Holding member for information storage disk and information storge disk drive device

ABSTRACT

An information storage disk holding member for holding an information storage disk in position in an information storage disk drive device, the holding member being made of glass-ceramics in which a crystal phase is dispersed in a glass matrix. The holding member has specific rigidity (Young&#39;s modulus/specific gravity) of not greater than 37 GPa, specific gravity of not greater than 3.0, coefficient of thermal expansion within a range from −50° C. to +70° C. which is within a range from +35×10 −7 /° C. to +130×10 −7 /° C., Young&#39;s modulus within a range from 95 GPa to 130 GPa, specific gravity within a range from 2.40 to 2.60 and bending strength within a range from 400 MPa to 800 MPa.  
     There is provided an information storage disk holding member which is capable of coping with a high speed rotation of a disk in conformity with tendencies toward high speed transmission of information, increasing mechanical strength for adaptation to mobile uses, having a thermal expansion property matching that of other drive component parts and eliminating defects caused by solving out of alkali. There is also provided an information storage disk drive device using this holding member.

TECHNICAL FIELD

[0001] This invention relates to information storage disk holdingmembers made of glass-ceramics and an information storage disk drivedevice using the disk holding members. In this specification, “holdingmembers” mean members for holding an information storage disk inposition in an information storage disk drive device and include, forexample, a rotor hub in the form of a rotating column for holding one ormore information storage disk, a disk supporting plane forming member onwhich information storage disks are stacked with a predeterminedinterval, a spacer ring and a shim, and a clamp as a mounting member.

BACKGROUND OF THE INVENTION

[0002] An information storage disk drive device is generally constructedin such a manner that one or more information storage disks and spacerrings are mounted alternately on a rotor hub which is secured fixedly toa rotor shaft and these disks and spacer rings are finally damped with ashim and a clamp. The holding members such as the rotor hub and spacerrings are normally made of metal materials such as stainless steel andan aluminum alloy. There is, however, an increasing tendency towardusing a disk substrate made of glass-ceramics in lieu of theconventional aluminum disk. In this case, use of a metal material forholding members is undesirable because there is a great difference inthe coefficient of thermal expansion between the holding members and thesubstrate and, moreover, there occur distortion, deformation and flashin the course of processing of the holding members. Furthermore, sincean aluminum alloy is a soft material, deformation tends to occur duringa high speed rotation and, therefore, it cannot cope sufficiently withcurrent requirement for a high density recording.

[0003] In this technical field, since a high precision is required forpositioning of a head and a medium, a particularly high precision insize is required for component parts of a medium substrate and a disk.For this reason, it is desirable for information storage disk holdingmembers to have as little difference as possible between other componentparts of a disk drive device (hereinafter referred to “other drivecomponent parts”) in the coefficient of thermal expansion within atemperature range of an environment in which the drive device is used.It is also desirable for them to have a low dust producing property, ahigh reliability, a low cost and a good polishing property. There isdemand for a more suitable material for information storage disk holdingmembers which satisfies all of these requirements.

[0004] As materials for solving these problems, there have been proposedpolycrystalline ceramics (Japanese Patent Application Laid-openPublication No. Sho 61-148667 and Japanese Patent Application Laid-openPublication No. Hei 9-44969 etc.) and amorphous glass (Japanese PatentApplication Laid-open Publication No. Hei 10-74350 etc.) butsatisfactory solution has not been achieved yet.

[0005] It is an object of the present invention to provide informationstorage disk holding members which have solved the above describedproblems and are capable of coping with a high speed rotation of a diskin conformity with tendencies toward high density recording and highspeed transmission of information, increasing mechanical strength foradaptation to mobile uses, having a thermal expansion property matchingthat of other drive component parts and eliminating defects caused bydissolving out of alkali, and also to provide an information storagedisk drive device using these holding members.

DISCLOSURE OF THE INVENTION

[0006] As a result of studies and experiments made by the inventors ofthe present invention for achieving the above described object of theinvention, it has been found, which has led to the present invention,that, by heat treating a base glass of a proper composition such as aSiO₂—Li₂O—K₂—P₂O₅—Al₂O₃—ZrO₂ glass under proper conditions,glass-ceramics having a high Young's modulus, a low specific gravity, acoefficient of thermal expansion, a high mechanical strength and a lowdust producing property and being thereby capable of coping with a highspeed rotation of a disk and very suitable for information storage diskholding members, particularly for spacer rings, can be obtained.

[0007] As the first aspect of the invention, the invention described inclaim 1 is an information storage disk holding member for holding aninformation storage disk in position, said holding member being made ofglass-ceramics in which a crystal phase is dispersed in a glass matrix.

[0008] The invention described in claim 2 is an information storage diskholding member as defined in claim 1 wherein specific rigidity (Young'smodulus/specific gravity) is not smaller than 37 GPa and specificgravity is not greater than 3.0.

[0009] The invention described in claim 3 is an information storage diskholding member as defined in claim 1 wherein Young's modulus is within arange from 95 GPa to 130 GPa and specific gravity is within a range from2.40 to 2.60.

[0010] The invention described in claim 4 is an information storage diskholding member as defined in claim 1 wherein bending strength is withina range from 400 MPa to 800 MPa.

[0011] The invention described in claim 5 is an information storage diskholding member as defined in claim 1 wherein the glass-ceramicscomprise, as a predominant crystal phase or phases, at least one crystalphase selected from the group consisting of lithium disilicate(Li₂O.2SiO₂), α-quartz (α-SiO₂), α-quartz solid solution (α-SiO₂ solidsolution), α-cristobalite (α-SiO₂) and α-cristobalite solid solution(α-SiO₂ solid solution).

[0012] The invention described in claim 6 is an information storage diskholding member as defined in claim 5 wherein the glass-ceramicscomprise, as a predominant crystal phase, lithium disilicate.

[0013] The invention described in claim 7 is an information storage diskholding member as defined in claim 1 wherein an amount of crystal oflithium disilicate in the glass-ceramics is 3-10 mass % and an averagecrystal grain diameter of the crystal phase is within a range from 0.01μm-0.05 μm.

[0014] The invention described in claim 8 is an information storage diskholding member as defined in claim 1 wherein the glass-ceramicscomprise, in mass % on oxide basis, SiO₂  70-79% Li₂O   8-12% K₂O   0-4%MgO   0-less than 2% ZnO   0-less than 2% P₂O₅ 1.5-3% ZrO₂ 1.5-7% Al₂O₃  3-9% Sb₂O₃ + As₃O₃   0-2%.

[0015] The invention described in claim 9 is an information storage diskholding member as defined in any of claims 1-8 wherein coefficient ofthermal expansion within a range from −50° C. to +70° C. is within arange from +65×10⁻⁷/° C. to +130×10⁻⁷/° C.

[0016] As the second aspect of the invention, the invention described inclaim 10 is an information storage disk holding member as defined inclaim 1 wherein the glass-ceramics comprises, as a predominant crystalphase, α-quartz (α-SiO₂) or α-quartz solid solution (α-SiO₂ solidsolution), an amount of the crystal phase is 3-35 mass %, and an averagecrystal grain diameter of the crystal phase is not greater than 0.10 μm.

[0017] The invention described in claim 11 is an information storagedisk holding member as defined in claim 10 wherein an average crystalgrain diameter of the entire predominant crystal phase of theglass-ceramics is not greater than 0.05 μm.

[0018] The invention described in claim 12 is an information storagedisk holding member as defined in claim 10 wherein the glass-ceramicsare substantially free of PbO.

[0019] The invention described in claim 13 is an information storagedisk holding member as defined in claim 10 wherein coefficient ofthermal expansion within a range from −50° C. to +70° C. is within arange from +95×10⁻⁷/° C. to +110×10⁻⁷/° C.

[0020] The invention described in claim 14 is an information storagedisk holding member as defined in claim 10 wherein the glass-ceramicscomprise, in mass % on oxide basis, SiO₂  70-77% Li₂O   5-less than 9%K₂O   2-5% MgO + ZnO + SrO + BaO   1-2% Y₂O₃ + WO₃ + La₂O₃ + Bi₂O₃  1-3% P₂O₅ 1.0-2.5% ZrO₂ 2.0-7% Al₂O₃   5-10% Na₂O   0-1% Sb₂O₃ + As₃O₃  0-2%.

[0021] The invention described in claim 15 is an information storagedisk holding member as defined in any of claims 10-14 wherein an amountof crystal of lithium disilicate in the glass-ceramics is within a rangefrom 15 mass % to 40 mass %.

[0022] As the third aspect of the invention, the invention described inclaim 16 is an information storage disk holding member as defined inclaim 1 wherein the glass-ceramics comprise, as a predominant crystalphase or phases, at least one crystal phase selected from the groupconsisting of cordierite (Mg₂Al₄Si₅O₁₈), cordierite solid solution(Mg₂Al₄Si₅O₁₈ solid solution), spinel, spinel solid solution, enstatite(MgSiO₃), enstatite solid solution MgSiO₃ solid solution), β-quartz(β-SiO₂), β-quartz solid solution (β-SiO₂ solid solution), magnesiumtitanate (MgTi₂O₅) and magnesium titanate solid solution (MgTi₂O₅ solidsolution).

[0023] The invention described in claim 17 is an information storagedisk holding member as defined in claim 16 wherein the glass-ceramicscomprise, in mass % on oxide basis, SiO₂  40-60% MgO  10-18% Al₂O₃ 10-less than 20% P₂O₅   0-4% B₂O₃   0-4% CaO 0.5-4% SrO   0-2% BaO  0-5% ZrO₂   0-5% TiO₂ 2.5-12% Bi₂O₃   0-6% Sb₂O₃   0-1% As₂O₃   0-1%Fe₂O₃   0-2%.

[0024] The invention described in claim 18 is an information storagedisk holding member as defined in claim 16 or 17 wherein coefficient ofthermal expansion within a range from −50° C. to +70° C. is within arange from +30×10⁻⁷/° C. to +65×10⁻⁷/° C.

[0025] As the fourth aspect of the invention, the invention described inclaim 19 is an information storage disk holding member as defined inclaim 1 wherein the glass-ceramics comprise, as a predominant crystalphase or phases, at least one crystal phase selected from the groupconsisting of enstatite (MgSiO₃), enstatite solid solution (MgSiO₃ solidsolution), magnesium titanate (MgTi₂O₅), magnesium titanate solidsolution (MgTi₂O₅ solid solution), spinel and spinel solid solution, theglass-ceramics comprise Al₂O₃ in an amount of less than 20 mass %, andthe glass-ceramics have Young's modulus within a range from 115 GPa to160 GPa.

[0026] The invention described in claim 20 is an information storagedisk holding member as defined in claim 19 wherein the glass-ceramicscomprise enstatite (MgSiO₃) or enstatite solid solution (MgSiO₃ solidsolution) as a crystal phase having the largest precipitation amountfirst phase).

[0027] The invention described in claim 21 is an information storagedisk holding member as defined in claim 19 wherein the glass ceramicscomprise magnesium titanate (MgTi₂O₅) or magnesium titanate solidsolution (MgTi₂O₅ solid solution) as a crystal phase having the largestprecipitation amount first phase).

[0028] The invention described in claim 22 is an information storagedisk holding member as defined in claim 20 wherein the glass-ceramicscomprise, as a crystal phase having a precipitation amount which issmaller than the precipitation amount of the first phase, at least onecrytal phase selected from the group consisting of magnesium titanate(MgTi₂O₅), magnesium titanate solid solution (MgTi₂O₅ solid solution),spinel and spinel solid solution. The invention described in claim 23 isan information storage disk holding member as defined in claim 21wherein the glass-ceramics comprise, as a crystal phase having aprecipitation amount which is smaller than the precipitation amount ofthe first phase, at least one crytal phase selected from the groupconsisting of enstatite (MgSiO₃), enstatite solid solution (MgSiO₃ solidsolution), spinel and spinel solid solution.

[0029] The invention described in claim 24 is an information storagedisk holding member as defined in claim 19 wherein the glass-ceramicsare substantially free of Li₂O, Na₂O and K₂O.

[0030] The invention described in claim 25 is an information storagedisk holding member as defined in claim 19 wherein the glass-ceramicscomprise, in mass % on oxide basis, SiO₂  40-60% MgO  10-20% Al₂O₃ 10-less than 20% CaO 0.5-4% SrO 0.5-4% BaO   0-5% ZrO₂   0-5% TiO₂exceeding 8% and up to 12% Bi₂O₃   0-6% Sb₂O₃   0-1% As₂O₃   0-1%.

[0031] The invention described in claim 26 is an information storagedisk holding member as defined in claim 19 wherein the glass-ceramicscomprise an element selected from P, W, Nb, La, Y and Pb in an amount ofup to 3 mass % on oxide basis and/or an element selected from Cu, Co,Fe, Mn, Cr, Sn and V in an amount of up to 2 mass % on oxide basis.

[0032] The invention described in claim 27 is an information storagedisk holding member as defined in claim 19 wherein coefficient ofthermal expansion within a range from −50° C. to +70° C. is within arange from +40×10⁻⁷/° C. to +60×10⁻⁷/° C.

[0033] The invention described in claim 28 is an information storagedisk holding member as defined in claim 19 wherein a crystal graindiameter of the respective crystal phases is within a range from 0.05 μm0.30 μm.

[0034] The invention described in claim 29 is an information storagedisk holding member as defined in any of claims 19-28 wherein Vickers'hardness is within a range from 700 to 850.

[0035] As the fifth aspect of the invention, the invention described inclaim 30 is an information storage disk holding member as defined inclaim 1 wherein a predominant crystal phase or phases of theglass-ceramics are at least one crystal phase selected from the groupconsisting of β-quartz, β-quartz solid solution, enstatite, enstatitesolid solution, forsterite and forsterite solid solution.

[0036] The invention described in claim 31 is an information storagedisk holding member as defined in claim 30 wherein the glass-ceramicscomprise Al₂O₃ in an amount within a range from 10 mass % to less than20 mass % on oxide basis and have Young's modulus (GPa)/ specificgravity within a range from 37 to 63.

[0037] The invention described in claim 32 is an information storagedisk holding member as defined in claim 30 wherein the glass-ceramicscomprise, in mass % on oxide basis, SiO₂   40-60% MgO   10-20% Al₂O₃  10-less than 20% P₂O₅ 0.5-2.5% B₂O₃    1-4% Li₂O  0.5-4% CaO  0.5-4%ZrO₂  0.5-5% TiO₂  2.5-8% Sb₂O₃ 0.01-0.5% As₂O₃    0-0.5% SnO₂    0-5%MoO₃    0-3% CeO    0-5% Fe₂O₃    0-5%.

[0038] The invention described in claim 33 is an information storagedisk holding member as defined in claim 30 wherein the glass-ceramicsare substantially free of Na₂O, K₂O and PbO.

[0039] The invention described in claim 34 is an information storagedisk holding member as defined in any of claims 30-33 wherein a crystalgrain diameter of the respective crystal phases is within a range from0.05 μm to 0.30 μm.

[0040] As the sixth aspect of the invention, the invention described inclaim 35 is an information storage disk holding member as defined inclaim 1 wherein the glass-ceramics comprise, as a predominant crystalphase or phases, at least one crystal phase selected from the groupconsisting of cordierite, cordierite solid solution, spinel, spinelsolid solution, enstatite, enstatite solid solution, β-quartz andβ-quartz solid solution.

[0041] The invention described in claim 36 is an information storagedisk holding member as defined in claim 1 wherein the glass-ceramicscomprise, as a predominant crystal phase or phases, at least one crystalphase selected from the group consisting of β-quartz β-quartz solidsolution, enstatite, enstatite solid solution, forsterite and forsteritesolid solution.

[0042] The invention described in claim 37 is an information storagedisk holding member as defined in claim 35 wherein a crystal graindiameter of the respective crystal phases is within a range from 0.05 μmto 0.30 μm.

[0043] The invention described in claim 38 is an information storagedisk holding member as defined in claim 35 wherein the glass-ceramicscomprise, in mass % on oxide basis, SiO₂  40-60% MgO  10-20% Al₂O₃ 10-less than 20% P₂O₅   0-4% B₂O₃   0-4% CaO 0.5-4% BaO   0-5% ZrO₂  0-5% TiO₂ 2.5-8% Sb₂O₃   0-1% As₂O₃   0-1% F   0-3% Fe₂O₃   0-5%.

[0044] The invention described in claim 39 is an information storagedisk holding member as defined in claim 35 wherein the glass-ceramicshave Young's modulus (GPa)/specific gravity within a range from 37 to 63and comprise Al₂O₃ within a range from 10% to less than 20%.

[0045] The invention described in claim 40 is an information storagedisk holding member as defined in claim 35 wherein the glass-ceramicsare substantially free of Na₂O, K₂O and PbO.

[0046] The invention described in claim 41 is an information storagedisk holding member as defined in any of claims 35-40 whereincoefficient of thermal expansion within a range from −50° C. to +70° C.is within a range from +30×10⁻⁷/° C. to +50×10⁻⁷/° C.

[0047] As the seventh aspect of the invention, the invention describedin claim 42 is an information storage disk holding member as defined inclaim 1 wherein the glass-ceramics comprise, as a predominant crystalphase or phases, at least one crystal phase selected from the groupconsisting of β-quartz (β-SiO₂), β-quartz solid solution (β-SiO₂ solidsolution), β-spodumene (β-Li₂O.Al₂O.SiO₂), β-spodumene solid solution(β-Li₂O.Al₂O₃.SiO₂ solid solution), β-eucryptite (β-Li₂O.Al₂O₃.2SiO₂where a part of Li₂O is replaceable by MgO and/or ZnO) and β-eucryptitesolid solution (β-Li₂O.Al₂O₃.2SiO₂ solid solution where a part of Li₂Ois replaceable by MgO and/or ZnO).

[0048] The invention described in claim 43 is an information storagedisk holding member as defined in claim 42 wherein an average crystalgrain diameter of the glass-ceramics is within a range from 0.001 μm to0.10 μm.

[0049] The invention described in claim 44 is an information storagedisk holding member as defined in claim 42 wherein the glass-ceramicscomprise, in mass % on oxide basis, SiO₂  50-62% P₂O₅   5-10% Al₂O₃ 22-26% Li₂O + MgO + ZnO   4-6.5% in which Li₂O   3-5% MgO 0.5-2% ZnO0.2-2% CaO + BaO 0.8-5% in which CaO 0.3-4% BaO 0.5-4% TiO₂   1-4% ZrO₂  1-4% As₂O₃ + Sb₂O₃   0-4%

[0050] and are substantially free of PbO, Na₂O and K₂O.

[0051] The invention described in claim 45 is an information storagedisk holding member as defined in any of claims 42-44 whereincoefficient of thermal expansion within a range from −50° C. to +600° C.is within a range from −10×10⁻⁷/° C. to +20×10⁻⁷/° C.

[0052] As the eighth aspect of the invention, the invention described inclaim 46 is an information storage disk holding member as defined inclaim 1 wherein a predominant crystal phase of the glass-ceramics isgahnite (ZnAl₂O₃) and/or gahnite solid solution (ZnAl₂O₃ solidsolution).

[0053] The invention described in claim 47 is an information storagedisk holding member as defined in claim 46 wherein the glass-ceramicsare substantially free of PbO, Na₂O and K₂O.

[0054] The invention described in claim 48 is an information storagedisk holding member as defined in claim 46 wherein the glass-ceramicscomprise, in mass % on oxide basis, SiO₂  30-65% Al₂O₃   5-35% ZnO  5-35% MgO   1-20% TiO₂   1-15% CaO + SrO + BaO + B₂O₃ + La₂O₃ +0.5-20% Y₂O₃ + Gd₂O₃ + Ta₂O₅ + Nb₂O₅ + WO₃ + Bi₂O₃ in which B₂O₃   0-10%and Ta₂O₅ + Nb₂O₅ + WO₃ + Bi₂O₃   0-10% ZrO₂ + P₂O₅ + SnO₂   0-7% inwhich ZrO₂   0-less than 2% P₂O₅   0-5% SnO₂   0-2% As₂O₃ + Sb₂O₃  0-4%.

[0055] The invention described in claim 49 is an information storagedisk holding member as defined in any of claims 46-48 whereincoefficient of thermal expansion within a range from −50° C. to +600° C.is within a range from +35×10⁻⁷/° C. to +65×10⁻⁷/° C.

[0056] As the ninth aspect of the invention, the invention described inclaim 50 is an information storage disk holding member as defined inclaim 1 wherein the glass-ceramics comprise, as a predominant crystalphase or phases, at least one crystal phase selected from the groupconsisting of α-cristobalite, α-cristobalite solid solution, α-quartzand α-quartz solid solution but are substantially free of lithiumdisilicate (Li₂O.2SiO₂), lithium silicate (Li₂O.SiO₂), β-spodumene,β-eucryptite, β-quartz, mica and fluorrichterite and also are free of Crand Mn, have a coefficient of thermal expansion within a range from −50°C. to +70° C. which is within a range from +65×10⁻⁷/° C. to +140×10⁻⁷/°C. and have an average crystal grain diameter of the predominant crystalphase of less than 0.10 μm.

[0057] The invention described in claim 51 is an information storagedisk holding member as defined in claim 50 wherein the glass-ceramicshave Young's modulus which is not smaller than 80 GPa.

[0058] The invention described in claim 52 is an information storagedisk holding member as defined in claim 50 wherein the glass-ceramicshave specific gravity within a range from 2.3 to 2.7.

[0059] The invention described in claim 53 is an information storagedisk holding member as defined in claim 50 wherein the glass-ceramicshave light transmittance for a plate thickness of 10 mm which is 90% orover within a wavelength range from 950 nm to 1600 nm.

[0060] The invention described in claim 54 is an information storagedisk holding member as defined in claim 50 wherein the glass-ceramicshave bending strength of 250 MPa or over.

[0061] The invention described in claim 55 is an information storagedisk holding member as defined in claim 50 wherein the glass-ceramicshave Vickers' hardness within a range from 600 to 800.

[0062] The invention described in claim 56 is an information storagedisk holding member as defined in any of claims 50-55 wherein theglass-ceramics comprise, in mass % on oxide basis, SiO₂  65-75% Li₂O  4-less than 7% K₂O   0-3% Na₂O   0-3% MgO + ZnO + SrO + BaO + CaO  2-15% Y₂O₃ + WO₃ + La₂O₃ + Bi₂O₃   0-3% SnO₂   0-3% P₂O₅ 1.0-2.5% ZrO₂2.0-7% Al₂O₃   5-9% Sb₂O₃ + As₂O₃   0-1%.

[0063] As other aspect of the invention, the invention described inclaim 57 is an information storage disk holding member as defined in anyof claims 1, 10 and 16 wherein the glass-ceramics are obtained bysubjecting base glass obtained by melting and forming glass rawmaterials to heat treatment for nucleation under a temperature within arange from 400° C. to 600° C. for one to seven hours and furthersubjecting the base glass to heat treatment for crystallization under atemperature within a range from 700° C. to 780° C. for one to sevenhours.

[0064] The invention described in claim 58 is an information storagedisk holding member as defined in any of claims 19, 30, 35, 36, 42 and46 wherein the glass-ceramics are obtained by subjecting base glassobtained by melting and forming glass raw materials to heat treatmentfor nucleation under a temperature within a range from 650° C. to 750°C. for one to seven hours and further subjecting the base glass to heattreatment for crystallization under a temperature within a range from750° C. to 950° C. for one to seven hours.

[0065] The invention described in claim 59 is an information storagedisk holding member as defined in claim 50 wherein the glass-ceramicsare obtained by subjecting base glass obtained by melting and formingglass raw materials to heat treatment for nucleation under a temperaturewithin a range from 400° C. to 600° C. for one to seven hours andfurther subjecting the base glass to heat treatment for crystallizationunder a temperature within a range from 650° C. to 750° C. for one toseven hours.

[0066] The invention described in claim 60 is an information storagedisk holding member made by forming a conductive film on the surface ofthe holding member as defined in any of claims 1, 10, 16, 19, 30, 35,36, 42, 46 and 50.

[0067] The invention described in claim 61 is a spacer ring for aninformation storage disk made of the holding member as defined in any ofclaims 1, 10, 16, 19, 30, 35, 36, 42, 46 and 50, said holding memberhaving a ring shape.

[0068] The invention described in claim 62 is an information storagedisk drive device capable of holding one or more information storagedisks on a rotor hub by means of the spacer ring as defined in claim 61.

[0069] The invention described in claim 63 is an information storagedisk drive device as defined in claim 62 wherein the rotor hub and thespacer ring have a coefficient of thermal expansion which issubstantially equal to that of the information storage disk.

[0070] The information storage disk holding member according to theinvention is made of glass-ceramics in which a crystal phase isdispersed in a glass matrix. By employing glass-ceramics as a materialfor the information storage disk holding member, high rigidity isachieved notwithstanding that specific gravity is relatively small andhigh specific rigidity thereby is achieved. By adopting the structure inwhich a crystal phase is dispersed in glass matrix, the glass-ceramicshave a very low dust generating capability. More specifically, theproblem of chipping which occurs in processing of the prior artamorphous glass material or polycrystalline ceramic material and theproblem of difference in thermal expansion relative to other drivecomponents can be eliminated. Further, the chemical strengtheningtreatment which is required for the amorphous glass material becomesunnecessary and, accordingly, advantages are brought about inproductivity and cost. The problems of production of particles due tofalling of crystal grains and generation of gas during mounting of aspacer ring which occurs in the polycrystalline ceramic material arealso eliminated.

[0071] As to the grain shape of the predominant crystal phase of theglass-ceramics of the present invention, if the grain shape isindefinite, excellent smoothness cannot be obtained and, further,falling of precipitated crystals tend to take place whereby particles ofcrystals cause damage to a magnetic head or a medium, the grain shapeshould preferably be as near to a sphere as possible and most preferablybe substantially spherical from the standpoint of surfacecharacteristics of the information storage medium surface.

[0072] Description will be made about a coefficient of thermal expansionof the information storage disk holding members of the present inventionand the spacer ring. As the recording density of theinformation-recording device increases, high accuracy is required forpositioning of a magnetic head and a medium and, accordingly, highaccuracy is required in the size of each component part of theinformation storage disk drive device. For this reason, influence ofdifference in the coefficient of thermal expansion relative to eachcomponent part cannot be ignored. It is therefore desirable to minimizedifference in the coefficient of thermal expansion relative to thesecomponent parts to the maximum extent. More specifically, it isdesirable for holding members such as a rotor hub and a spacer ring tohave a coefficient of thermal expansion which is substantially equal tothat of the information storage disk. Strictly speaking, there is a casewhere it is desirable that a coefficient of thermal expansion of theseholding members should be only slightly smaller than that of theinformation storage disk. Particularly, as the coefficient of thermalexpansion of the component parts of the information storage disk drivedevice, a coefficient of about +90×10⁻⁷/° C. to +110×10⁻⁷/° C. is mostfrequently used. Accordingly, for coping broadly with material of acomponent part employed, the coefficient of thermal expansion of theinformation storage disk holding members of the invention, particularlythat of the spacer ring, should preferably be within a range from+65×10⁻⁷/° C. to +130×10⁻⁷/° C. within temperature range from −50° C. to+70° C. in which the they are used.

[0073] In the third aspect of the invention, for coping broadly withmaterial of a component part employed while considering balance withstrength of the in crystal phase of the present invention, thecoefficient of thermal expansion of the glass-ceramics should preferablybe within a range from +65×10⁻⁷/° C. to +110×10⁻⁷/° C. withintemperature range from −50° C. to +70° C. The coefficient should morepreferably be not smaller than +95×10⁻⁷/° C. and not greater than+110×10⁻⁷/° C.

[0074] In a case where an information storage disk substrate having arelatively smaller coefficient of thermal expansion than the onesdescribed above, for example, in a case where a disk substrate made of aglass-ceramic having a coefficient of thermal expansion within a rangefrom +30×10⁻⁷/° C. to +60×10⁻⁷/° C. within a range from −50° C. to +70°C. is used, the coefficient of thermal expansion of the informationstorage disk holding members of the present invention should preferablybe within a range from +30×10⁻⁷/° C. to +less than 65×10⁻⁷/° C. within arange from −50° C. to +70° C. Particularly in the fourth aspect of theinvention, it should more preferably be within a range from +40×10⁻⁷/°C. to +60×10⁻⁷/° C. and in the fifth and sixth aspects of the invention,it should more preferably be within a range from +30×10⁻⁷/° C. to+50×10⁻⁷/° C.

[0075] In the seventh and eighth aspects of the invention, thecoefficient of thermal expansion of the information storage disk holdingmembers can be set within a range from −10×10⁻⁷/° C. to +80×10⁻⁷/° C.within a range from −50° C. to +70° C. Particularly in the seventhaspect of the invention, it should preferably be within a range from−10×10⁻⁷/° C. to +20×10⁻⁷/° C. and, more preferably, be within a rangefrom −10×10⁻⁷/° C. to +10×10⁻⁷/° C. from the standpoint of harmonizationwith other physical properties. In the eighth aspect of the invention,it should preferably be within a range from +30×10⁻⁷/° C. to +80×10⁻⁷/°C. and, more preferably, be within a range from +35×10⁻⁷/° C. to+65×10⁻⁷/° C.

[0076] In the ninth aspect of the invention, for coping broadly withmaterial of a component part used, the coefficient of thermal expansionof the information storage disk holding members should preferably bewithin a range from +60×10⁻⁷/° C. to +135×10⁻⁷/° C. within a range from−50° C. to +70° C. The average linear coefficient of thermal expansionshould preferably be not smaller than +70×10⁻⁷/° C. and, more preferablybe not greater than +120×10⁻⁷/° C.

[0077] Description will now be made about Young's modulus, specificgravity and mechanical strength of the information storage disk holdingmembers, particularly those of the spacer ring.

[0078] As to Young's modulus and specific gravity, high rigidity and lowspecific gravity are preferable for coping with increase in the speed oftransmitting information, i.e., high speed rotation of 10000 rpm orover. That is, it is necessary to balance high rigidity and low specificgravity which are characteristics contradicting to each other inappearance. A large specific gravity tends to cause vibration duringhigh speed rotation even if Young's modulus is high. Specific rigidity(Young's modulus/specific gravity) of the information storage diskholding members of the invention, therefore, should preferably be notsmaller than 37 GPa, more preferably not smaller than 38 GPa and mostpreferably not smaller than 39 GPa. From the standpoint ofprocessability in polishing, the specific rigidity should preferably benot greater than 63 GPa, more preferably be not greater than 57 GPa andmost preferably be not greater than 54 GPa. In the fifth and sixthaspects of the invention, a preferable range of the specific rigidity(Young's modulus/specific gravity) of the information storage diskholding members is 40-63 GPa, a more preferable range thereof is 47-63GPa and the most preferable range thereof is 50-63 GPa. In the ninthaspect of the invention, the specific rigidity (Young's modulus/specificgravity) of the information storage disk holding members shouldpreferably be 30-65 GPa, and more preferably be 33-60 GPa.

[0079] For the same reason, Young's modulus of the information storagedisk holding members of the invention, particularly that of the spacerring, should preferably be not smaller than 80 GPa, more preferably benot smaller than 85 GPa and most preferably be not smaller than 95 GPaand the specific gravity should preferably be not greater than 2.50.From the standpoint of adaptability with other component parts, Young'smodulus of the information storage disk holding members of the inventionshould preferably be within a range of 95-130 GPa and, more preferably,be within a range of 95-110 GPa. Particularly, in the fourth, fifth andsixth aspects of the invention, Young's modulus may be a value notsmaller than 115 GPa and not greater than 150 GPa.

[0080] In the fourth aspect of the invention, the specific gravity ofthe information storage disk holding members should preferably be notgreater than 3.0 and, more preferably be within a range of 2.4-2.60 and,more preferably be within a range of 2.40-2.50. In the fifth and sixthaspects of the invention, a preferable range of the specific gravity ofthe information storage disk holding members is 2.5-3.3. For havingsufficient shock proof property in uses such as for mobiles, bendingstrength of the information storage disk holding members of theinvention should preferably be not smaller than 250 MPa, more preferablybe not smaller than 400 MPa and most preferably be not smaller than 500MPa. From the standpoint of designing composition of glass-ceramics, thebending strength should preferably be not greater than 800 MPa.

[0081] As a high Young's modulus of a material is adopted, surfacehardness of the material generally tends to increase. The surfacehardness of the substrate (Vickers' hardness) should preferably be notsmaller than 600 (5880 N/mm²), more preferably be not smaller than 650(6370 N/mm²) and, most preferably, be not smaller than 700 (6860 N/mm²).On the other hand, if it is too hard, processing time in polishingbecomes excessively long with resulting deterioration in productivityand cost. Considering productivity caused by processability, the surfacehardness of the substrate (Vickers' hardness) should preferably be notgreater than 850 (8330 N/mm²), more preferably be not greater than 800(7840 N/mm²) and, most preferably, be not greater than 760 (7448 N/mm²).

[0082] For making it possible to hold the information storage disk in apredetermined position when the disk is rotating at a high speed,flatness of the surface of the information storage disk holding memberswhich is in contact with the information storage disk should preferablybe not greater than 5 μm, more preferably be not greater than 3 μm and,most preferably, be not greater than 1 μm. Mean surface roughness at thecenter line (Ra) should preferably be within a range of 0.1-2.0 μm.

[0083] It is difficult to realize polishing processability andparticularly excellent mechanical strength, particularly bendingstrength unless crystal grain diameter is controlled. The crystal graindiameter should preferably be within a range of 0.001-0.10 μm, morepreferably be within a range of 0.001-0.07 μm and, most preferably, bewithin a range of 0.001-0.05 μm.

[0084] Description will now be made about preferable crystal phases andcompositions of glass-ceramics which constitute the holding members ofthe invention.

[0085] In the present specification, amount of crystal means ratio (mass%) of a specific crystal phase to the entire mass of a glass-ceramic.Degree of crystallization means ratio (mass %) of entire crystal phaseor phases to the entire mass of a glass-ceramic. Compositions areexpressed on oxide basis. In the present specification, a predominantcrystal phase means all crystal phase or phases each of which has arelatively large precipitation ratio. More specifically, when the mainpeak (the highest peak) of a crystal phase which has the largest ratioof precipitation in an X-ray chart of X-ray diffraction (the verticalaxis represents X-ray diffraction intensity and the horizontal axisrepresents diffraction angle) is taken as 100, any crystal phase whoseratio of X-ray diffraction intensity (hereinafter referred to as “X-rayintensity ratio) of the main peak (the highest peak in this crystalphase) is 30 or more is referred to as “predominant crystal phase”.

[0086] Preferable crystal phases and compositions of the glass-ceramicsconstituting the holding members of the first aspect of the inventionwill be described below.

[0087] In the first aspect of the invention, the glass-ceramicsconstituting the holding members of the invention should preferablycomprise, as a predominant crystal phase or phases, at least one crystalphase selected from the group consisting of lithium disilicate(Li₂O.2SiO₂), α-quartz (α-SiO₂), α-quartz solid solution (α-SiO₂ solidsolution), α-cristobalite (α-SiO₂) and α-cristobalite solid solution(α-SiO₂ solid solution). The predominant crystal phase is an importantfactor determining the coefficient of thermal expansion and mechanicalstrength and, for realizing the properties required for the abovedescribed holding members, particularly the spacer ring, thesepredominant crystal phases are suitable.

[0088] In the first aspect of the invention, the glass-ceramics shouldpreferably comprise, as a predominant crystal phase, lithium disilicateand an amount of crystal of lithium disilicate in the glass-ceramicsshould preferably be 3-10 mass % and an average crystal grain diameterof the crystal phase should preferably be within a range from 0.01μm-0.0605 μm.

[0089] In the first aspect of the invention, the glass-ceramics shouldpreferably comprise, as a predominant crystal phase, α-quartz orα-quartz solid solution and an amount of crystal phase of α-quartz andα-quartz solid solution in the glass-ceramics should preferably be 5-25mass % and an average crystal grain diameter thereof should preferablybe 0.01 μm 0. 10 μm.

[0090] In the first aspect of the invention, the glass-ceramics shouldpreferably comprise, as a predominant crystal phase, α-cristobalite orα-cristobalite solid solution and an amount of crystal phase ofα-cristobalite and α-cristobalite solid solution in the glass-ceramicsshould preferably be 2-10 mass % and an average crystal grain diameterthereof should preferably be 0.01 μm-010 μm.

[0091] Reasons for defining the composition range of the base glass asdescribed M above will now be described.

[0092] The SiO₂ ingredient is a very important ingredient which, by heattreatment of the base glass, produces lithium disilicate (Li₂O.2SiO₂),α-quartz (α-SiO₂), α-quartz solid solution (α-SiO₂ solid solution),α-cristobalite (α-SiO₂) or α-cristobalite solid solution (α-SiO₂ solidsolution) as a predominant crystal phase. For maintaining stability ofprecipitating crystals of the glass-ceramics obtained and preventingtheir texture from becoming coarse, an amount of this ingredient shouldpreferably be not smaller than 70% and, for maintaining excellentmelting property and formability of the base glass, the amount of thisingredient should preferably be not greater than 79% and, morepreferably be not greater than 77%.

[0093] The Li₂O ingredient is a very important ingredient which produceslithium disilicate (Li₂O.2SiO₂). If the amount of this ingredient isless than 8%, difficulty arises in precipitation of this crystal andalso in melting of the base glass. The amount of this ingredient,therefore, should preferably be not smaller than 8%. If the amount ofthis ingredient exceeds 12%, stability of the obtained crystal isdeteriorated and its texture becomes coarse and, moreover, chemicaldurability also is deteriorated. The amount of this ingredient,therefore, should preferably be not greater than 12%.

[0094] The K₂O ingredient improves the melting property of the glass andprevents the precipitating crystal from becoming coarse. If the amountof this ingredient exceeds 4%, the precipitating crystal sometimesbecomes coarse, the crystal phase changes and chemical durability isdeteriorated and, therefore, the amount of this ingredient shouldpreferably be not greater than 4% and, more preferably, should be withina range of 1-3%.

[0095] The MgO and ZnO ingredients improve the melting property of theglass and prevent the precipitating crystal from becoming coarse and,further, are effective for causing the crystal grains of lithiumdisilicate (Li₂O.2SiO₂), α-quartz (α-SiO₂), β-quartz solid solution(α-SiO₂ solid solution), α-cristobalite (α-SiO₂) or α-cristobalite solidsolution (α-SiO₂ solid solution) to precipitate in spherical shape. TheMgO ingredient up to 2% and the ZnO ingredient up to 2% will suffice.

[0096] In the present invention, the P₂O₅ ingredient is indispensable asa nucleating agent and, for enhancing forming of crystal nucleuses andpreventing a predominant crystal phase from becoming coarse, the amountof this ingredient should preferably be not smaller than 1.5%. Forpreventing the base glass from becoming opaque and maintaining stabilityin a large scale production, the amount of this ingredient shouldpreferably be not greater than 3%.

[0097] The ZrO₂ ingredient is a very important ingredient which, likethe P₂O₅ ingredient, functions as a nucleating agent and, moreover, hasbeen found to have a remarkable effect for making precipitating crystalsfine and improve mechanical strength and chemical durability of thematerial. For obtaining such effect, the amount of the ZrO₂ ingredientshould preferably be not smaller than 1.5% and, more preferably, be notsmaller than 2%. If the amount of this ingredient exceeds 9%, melting ofthe base glass becomes difficult and substance such as ZrSiO₄ which isleft unmelted is produced and, therefore, the amount of this ingredientshould preferably be not greater than 9% and, more preferably, be notgreater than 7%.

[0098] The Al₂O₃ ingredient improves chemical durability and hardness ofthe glass-ceramics and the amount of this ingredient should preferablybe not smaller than 3%. If the amount of this ingredient exceeds 9%, themelting property and resistance to devitrification deteriorate and thepredominant crystal phase change to β-spodumene (Li₂O.Al₂O_(3.) 4SiO₂)which is a low expansion crystal and, therefore, the amount of thisingredient should preferably be not greater than 9%. In the compositionsof the present invention, precipitation of β spodumene(Li₂O.Al₂O₃.4SiO₂) and β-cristobalite (β-SiO₂) deteriorates thecoefficient of thermal expansion of the material significantly and,therefore, precipitation of these crystals should preferably be avoided.

[0099] The Sb₂O₃ and/or As₂O₃ ingredients may be added as a refiningagent in melting glass and it will suffice if 2% or below of theseingredients is added.

[0100] Description will now be made about preferable crystal phases andcompositions of the glass-ceramics which constitute the holding membersof the second aspect of the invention.

[0101] In the second aspect of the invention, the glass-ceramics whichconstitute the information storage disk holding members shouldpreferably contain lithium disilicate (Li₂O.2SiO₂) as a predominantcrystal phase and the amount of Li₂O should preferably be within a rangefrom 5% to less than 9% on oxide basis. In the second aspect of theinvention, the amount of the crystal phase of lithium disilicate shouldpreferably be within a range of 15-40%.

[0102] In the second aspect of the invention, the glass-ceramics shouldpreferably contain (1) lithium disilicate (Li₂O.2SiO₂) and (2) α-quartz(α-SiO₂) or α-quartz solid solution (α-SiO₂ solid solution) aspredominant crystal phases and the average crystal grain diameter of thecrystal phases as a whole should preferably be not greater than 0.05 μm.

[0103] In the second aspect of the invention, the glass-ceramics shouldpreferably contain (1) lithium disilicate (Li₂O.2SiO₂) and (2) α-quartz(α-SiO₂) or α-quartz solid solution (α-SiO₂ solid solution) aspredominant crystal phases and the amount of crystal phases of α-quartz(α-SiO₂) or α-quartz solid solution (α-SiO₂ solid solution) shouldpreferably be within a range of 3-35% and the average crystal graindiameter should preferably be not greater than 0.10 μm.

[0104] The crystal grain of the glass-ceramics which constitute theinformation storage disk holding members of the invention shouldpreferably be fine and substantially spherical.

[0105] In the second aspect of the invention, the glass-ceramics shouldpreferably comprise, in mass % on oxide basis, SiO₂  70-77% Li₂O  5-less than 9% K₂O   2-5% MgO + ZnO + SrO + BaO   1-2% Y₂O₃ + WO₃ +La₂O₃ + Bi₂O₃   1-3% P₂O₅ 1.0-2.5% ZrO₂ 2.0-7% Al₂O₃   5-10% Na₂O   0-1%Sb₂O₃ + As₃O₃   0-2%.

[0106] The base glass comprising the above described composition is heattreated at 400° C.-600° C. for 1 to 7 hours for nucleation and isfurther treated at 700° C.-760° C. for 1 to 7 hours for crystallizationthereby to produce the glass-ceramics of the second aspect of theinvention.

[0107] Description will be made about Young's modulus, specific gravityand mechanical strength of the glass-ceramics of the second aspect.

[0108] As to Young's modulus and specific gravity, for coping with highspeed rotation of an information storage disk in correspondence to highspeed transmission of information, the glass-ceramics should preferablyhave high rigidity and low specific gravity. Even if they have highrigidity, if they have low specific gravity, deflection takes placeduring high speed rotation due to their large weight which causesvibration. Conversely, vibration takes place similarly if they have lowrigidity even when they have low specific gravity. Therefore, it isnecessary to balance high rigidity and low specific gravity which arecharacteristics contradicting to each other in appearance. A preferablerange of Young's modulus (GPa)/specific gravity is 37 or over. A morepreferable range thereof is 39 or over, an even more preferable rangethereof is 41 and the most preferable range thereof is 43 or over. Thereis also a preferable range of rigidity. Even when the glass-ceramicshave low specific gravity and the above described range is satisfied,Young's modulus of the substrate should preferably be not smaller than95 GPa from the standpoint of generation of vibration. In the examplesof the second aspect, the glass-ceramics have Young's modulus within arange of 95 GPa-120 GPa. Similarly as to specific gravity, even when theglass-ceramics have high rigidity, the specific gravity shouldpreferably be not greater than 2.60 and, more preferably, not greaterthan 2.57 from the standpoint of generation of vibration. In theexamples of the second aspect of the invention, the glass-ceramics havespecific gravity within a range of 2.40-2.60.

[0109] Reasons for limiting the predominant crystal phases andcompositions of the glass-ceramics of the second aspect of the inventionwill be described below.

[0110] As to the predominant crystal phase of the glass-ceramics of thesecond aspect of the invention, since an information storage diskholding member comprising lithium disilicate has no crystal anisotropy,has little impurities and has a dense, uniform and fine texture, it canrealize high mechanical strength, processability, and capability ofcontrolling the coefficient of thermal expansion and high chemicaldurability and therefore is very useful. By containing further α-quartzor α-quartz solid solution as a the predominant crystal phase, bendingstrength can be further increased and, simultaneously, the coefficientof thermal expansion in −50° C.-+70° C. can be set at a higher leveland, hence, the information storage disk holding members containing (1)lithium disilicate and (2) α-quartz or α-quartz solid solution arepreferable because they have excellent mechanical strength, capabilityof controlling the coefficient of thermal expansion and chemicaldurability.

[0111] As to the amount of Li₂O, this ingredient is a very importantingredient which facilitates manufacture and melting of the base glassand causes lithium disilicate to precipitate. Conventionally, since theLi₂O ingredient has been added in an amount which is larger thanstoichiometrically necessary for constituting lithium disilicate, ansuperfluous amount of Li₂O which does not contribute to production ofthis crystal phase exists in the glass phase. This becomes a dissolvingalkali ingredient which causes the previously described problem.

[0112] More specifically, from the standpoint of the dissolving alkaliamount, glass-ceramics have less dissolving alkali amount than amorphousglass as described previously but it is desirable to further decreasethe amount of dissolving alkali amount in view of the high recordingdensity tendency. Since dissolving of alkali ingredient inglass-ceramics is mainly caused by dissolving from the amorphous portionof the matrix, it is desirable to decrease alkali ingredient other thanthat in the crystal to the maximum extent possible. As a result ofdetailed study and experiments, it has become apparent that, if theamount of dissolving out alkali from the surface of a substrate in analkali dissolving test is 1.0 μg/disk or over in a 2.5 inch disksubstrate (having the size of outer diameter of 65 mm, inner diameter of20 mm and thickness of 0.635 mm and being chamfered by 0.1 mm atchamfering angle of 45°), i.e., the amount of dissolving out alkali perunit area is 0.016 μg/mm² or over, magnetic characteristics are reduceddue to dispersion of alkali during the film forming process and alkalicompounds are generated by alkali ingredient which has dispersed up tothe surface of the recording medium whereby reading error and head crashtake place in a magnetic disk for which, for example, high speedrotation of 10000 rpm or over is required. The amount of dissolving outalkali should more preferably be not greater than 0.011 μg/cm² (notgreater than 0.7 μg/disk in a 2.5 inch disk substrate) and, mostpreferably, be not greater than 0.008 μg/mm² (ot greater than 0.5μg/disk in a 2.5 inch disk).

[0113] Accordingly, for containing Li₂O in an amount necessary forconstituting lithium disilicate and reducing a superfluous amount ofLi₂O which causes dissolving of alkali to the maximum extent possible,it is desirable to control the amount of Li₂O in a lower amount than theupper limit of the Li₂O amount of the conventional glass-ceramics whichcontain lithium disilicate as a predominant crystal phase. If the amountof this ingredient is less than 5%, precipitation of this crystalbecomes difficult and melting of the base glass becomes also difficult.If the amount of this ingredient is 9% or over, the crystal obtained isnot stable, its texture becomes coarse and chemical durability isdeteriorated and, moreover, a large amount of superfluous Li₂Oingredient which exceeds the stoichiometric amount which constitutes thecrystal is left in the glass matrix resulting in dissolving of Li ion.For obtaining holding members of better glass-ceramics, particularlythose having an amount of dissolving out alkali which is not greaterthan 0.011 μg/cm², the amount of Li₂O should preferably be within arange of 5% less than 8% and, most preferably, within a range of 5%-7%.

[0114] The above limitation concerning the range of the amount of Li₂Ois effective also for other crystal phases which requires Li₂O as aningredient for constituting a predominant crystal phase (e.g., crystalphases of spodumene, eucryptite and petalite). Since, however,possibility is high that a desired coefficient of thermal expansioncannot be obtained from β-spodumene, β-eucryptite or β-cristobalite(β-SiO₂) which has a negative thermal expansion characteristic, it ispreferable not to contain such crystal phase having a negative thermalexpansion characteristic.

[0115] As the amount of crystal of lithium disilicate, for achieving theabove described desired mechanical strength, processability, thermalexpansion characteristic and chemical durability and also reducing asuperfluous amount of Li₂O in the glass matrix in the above describedcontent range of Li₂O for eliminating the problem of dissolving ofalkali, a preferable range is 15%-40%, a more preferable range is20%-40% and the most preferable range is 20%-38%.

[0116] As to the crystal of α-quartz or α-quartz solid solution, forachieving a desired value in the control of mechanical strength(particularly bending strength) and thermal expansion characteristic,the amount of crystal should preferably be within a range of 3%-35% andmore preferably within a range of 5%-35%.

[0117] For realizing polishing capability and particularly excellentmechanical strength, particularly bending strength, it is difficult torealize such properties unless the crystal grain diameter must becontrolled. It is desirable to control the average crystal graindiameter of the crystal phase as a whole including lithium disilicate toa value not greater than 0.05 μm. Accordingly, in a case where thepredominant crystal phases include (1) lithium disilicate and (2)α-quartz or α-quartz solid solution, it is desirable to control theaverage crystal grain diameter of the predominant crystal phases as awhole to a value not greater than 0.05 μm. As to (2) α-quartz orα-quartz solid solution, the average crystal grain diameter may be setto a value not greater than 0.10 μm depending upon the shape of thecrystal grain and other factors. A preferable range of the averagecrystal grain diameter of the entire crystal phases is a value notgreater than 0.05 μm, that of lithium disilicate crystal is a value notgreater than 0.04 μm and that of α-quartz or α-quartz solid solutioncrystal is a value not greater than 0.07 μm. A more preferable range ofthe average crystal grain diameter of the entire crystal phases is avalue not greater than 0.03 μm, that of lithium disilicate crystal is avalue not greater than 0.03 μm and that of α-quartz or α-quartz solidsolution crystal is a value not greater than 0.05 μm.

[0118] Reasons for defining the composition range of the respectiveingredients other than Li₂O in the base glass as described above in thesecond aspect of the invention will now be described.

[0119] The SiO₂ ingredient is a very important ingredient which, by heattreatment of the base glass, produces lithium disilicate (Li₂O.2SiO₂),α-quartz (α-SiO₂) and α-quartz solid solution (α-SiO₂ solid solution) asa predominant crystal phase. If the amount of this ingredient is lessthan 70%, precipitated crystal of the glass-ceramics obtained becomesinstable and its texture tends to become coarse. If the amount of thisingredient exceeds 77%, difficulty arises in melting and forming of thebase glass.

[0120] The K₂O ingredient improves the melting property of the glass andprevents the precipitating crystal from becoming coarse. The amount ofthis ingredient should preferably be not smaller than 2%. If the amountof this ingredient is excessive, the precipitating crystal becomescoarse, the crystal phase changes and chemical durability isdeteriorated and, therefore, the amount of this ingredient shouldpreferably be not greater than 5%. Particularly in the presentinvention, dispersion of alkali ion can be prevented by mixing K₂O withLi₂O.

[0121] The MgO, ZnO, SrO and BaO ingredients improve the meltingproperty of the glass and prevent the precipitating crystal frombecoming coarse and, further, are effective for causing the crystalgrains of lithium disilicate (Li₂O.2SiO₂), α-quartz (α-SiO₂) andα-quartz solid solution (α-SiO₂ solid solution) to precipitate inspherical shape. For these purposes, the total amount of theseingredients should preferably be not smaller than 1.0%. If, however, thetotal amount of these ingredients exceeds 2%, crystals obtained becomeinstable and their texture tends to become coarse.

[0122] In the present invention, the P₂O₅ ingredient is indispensable asa nucleating agent and, for enhancing forming of crystal nucleuses andpreventing a predominant crystal phase from becoming coarse, the amountof this ingredient should preferably be not smaller than 1.0%. Forpreventing at the base glass from becoming opaque and maintainingstability in a large scale production, the amount of this ingredientshould preferably be not greater than 2.5%.

[0123] The ZrO₂ ingredient is a very important ingredient which, likethe P₂O₅ ingredient, functions as a nucleating agent and, moreover, hasbeen found to have a remarkable effect for making precipitating crystalsfine and improve mechanical strength and chemical durability of thematerial. For obtaining such effect, the amount of the ZrO₂ ingredientshould preferably be not smaller than 2.0%. If an excessive amount ofthis ingredient is added, melting of the base glass becomes difficultand substance such as ZrSiO₄ which is left unmelted is produced and,therefore, the amount of this ingredient should preferably be notgreater than 7%.

[0124] The Al₂O₃ ingredient improves chemical durability and mechanicalstrength, particularly hardness, of the glass-ceramics and the amount ofthis ingredient should preferably be not smaller than 5%. If the amountof this ingredient is excessive, the melting property and resistance todevitrification deteriorate and the predominant crystal phase change toβ-spodumene (Li₂O.Al₂O₃.4SiO₂) which is a low expansion crystal. Asdescribed above, precipitation of β-spodumene (Li₂O.Al₂O₃.4SiO₂)deteriorates the coefficient of thermal expansion significantly and,therefore, precipitation of this ingredient should be avoided. For thisreason, the amount of this ingredient should preferably be not greaterthan 10%.

[0125] The Y₂O₃, WO₃, La₂O₃ and Bi₂O₃ ingredients are importantingredients which improve the melting property which is reduced in acomposition comprising a low content of Li₂O and also increases Young'smodulus of the glass. If the total amount of these ingredients is lessthan 1%, these effects cannot be achieved whereas if the total amount ofthese ingredients exceeds 3%, precipitation of a stable crystal becomesdifficult. The Bi₂O₃ ingredient brings about coloring aftercrystallization by coexistence with As₂O₃ and this is effective forfacilitating finding of a scar and scratch and enhancing laser energyabsorption efficiency in laser texturing.

[0126] The Na₂O ingredient improves, like the K₂O ingredient, themelting property of the glass and prevents the precipitating crystalfrom becoming coarse and, further, by mixing with the Li₂O ingredient,prevents dispersion of alkali ion. These effects of this ingredient arenot so remarkable as the K₂O ingredient and, if the amount of thisingredient exceeds 1%, dissolving of Na ion increases rather thandecreases and, therefore, the amount of this ingredient shouldpreferably be not greater than 1%.

[0127] The Sb₂O₃ and/or As₂O₃ ingredients may be added as a refiningagent in melting glass and it will suffice if 2% or below, preferably 1%or below, of these ingredients is added.

[0128] In addition to the above described composition, one or moreelements selected from the group consisting of Cu, Co, Fe, Mn, Cr, Snand V may be added in an amount up to 2 weight % on oxide basis.

[0129] Preferable crystal phases and compositions of the glass-ceramicswhich constitute the holding members of the third aspect of theinvention will now be described.

[0130] In the third aspect of the invention, the glass-ceramics whichconstitute the holding members should preferably comprise, as apredominant crystal phase or phases, at least one crystal phase selectedfrom the group consisting of cordierite (Mg₂Al₄Si₅O₁₈), cordierite solidsolution Mg₂Al₄Si₅O₁₈ solid solution), spinel, spinel solid solution,enstatite (MgSiO₃), enstatite solid solution (MgSiO₃ solid solution),β-quartz (β-SiO₂), β-quartz solid solution (β-SiO₂ solid solution),magnesium titanate (MgTi₂O₅) and magnesium titanate solid solution(MgTi₂O₅ solid solution). This is because these crystal phases areadvantageous in that they have excellent processability, contribute toincrease in rigidity, have capability of making the grain diameter ofprecipitating crystal relatively small and have capability of reducingspecific gravity significantly compared with other crystal phases.

[0131] Spinel herein means a spinel type crystal including, e.g., (Mgand/or Zn)Al₂O₄, and (Mg and/or Zn)₂TiO₄ and a solid solution betweenthese crystals. Spinel solid solution herein means a solid solutioncrystal in which an ingredient or ingredients other than such spineltype crystal partly replaces or enters such spinel type crystal.

[0132] Description will be made about the crystal grain size ofcordierite (Mg₂Al₄Si₅O₁₈), spinel, enstatite (MgSiO₃), β-quartz (β-SiO₂)and magnesium titanate ((MgTi₂O) and solid solutions of these crystals.For obtaining a smooth surface which is suitable for an informationstorage disk holding member, average crystal grain diameter of thesepredominant crystal phases should preferably be not greater than 1.0 μmand, more preferably, be not greater than 0.5 μm.

[0133] In the information storage disk holding members of the thirdaspect of the invention, the glass-ceramics should preferably containthe crystal phase of cordierite or enstatite having an average crystalgrain diameter within a range of 0.10 μm-1.0 μm. More preferably, theglass-ceramics should contain the crystal phase of cordierite orenstatite having an average crystal grain diameter within a range of0.30 μm-1.0 μm. The amount of crystal of the cordierite crystal phaseshould preferably be within a range of 10-70 mass % and, morepreferably, within a range of 30-70 mass %. The amount of crystal of theenstatite crystal phase should preferably be within a range of 10-70mass % and, more preferably, within a range of 30-70 mass %.

[0134] Description will be made about a preferable composition range(oxide basis) of the glass-ceramics which constitute the informationstorage disk holding members of the third aspect of the invention. Inthis aspect, the glass-ceramics should preferably comprise, in mass %,SiO₂  40-60% MgO  10-18% Al₂O₃  10-less than 20% P₂O₅   0-4% B₂O₃   0-4%CaO 0.5-4% SrO   0-2% BaO   0-5% ZrO₂   0-5% TiO₂ 2.5-12% Bi₂O₃   0-6%Sb₂O₃   0-1% As₂O₃   0-1% Fe₂O₃   0-2%.

[0135] The SiO₂ ingredient is a very important ingredient which, by heattreatment of the base glass, produces cordierite (Mg₂Al₄Si₅O₁₈),cordierite solid solution (Mg₂Al₄Si₅O₁₈ solid solution), enstatite(MgSiO₃), enstatite solid solution (MgSiO₃ solid solution), β-quartz(β-SiO₂) and β-quartz solid solution (β-SiO₂ solid solution) aspredominant crystal phases. If the amount of the SiO₂ ingredient is lessthan 40%, the crystal phase of the obtained glass-ceramics is not stableand its texture becomes coarse whereas if the amount of this ingredientexceeds 60%, melting and forming of the base glass become difficult. Forprecipitation of these crystal phases, conditions of heat treatment arealso an important factor and a more preferable range of this ingredientfor enabling broader heat treatment conditions is 48.5-58.5%.

[0136] The MgO ingredient is a very important ingredient which, by heattreatment of the base glass, produces cordierite (Mg₂Al₄Si₅O₁₈),cordierite solid solution (Mg₂Al₄Si₅O₁₈ solid solution), spinel, spinelsolid solution, enstatite MgSiO₃), enstatite solid solution (MgSiO₃solid solution), β-quartz (β-SiO₂) and β-quartz solid solution (β-SiO₂solid solution) as predominant crystal phases. If the amount of thisingredient is less than 10%, the desired crystal cannot be obtained, thecrystal phase of the obtained glass-ceramics is not stable and itstexture becomes coarse and, moreover, the melting property deteriorateswhereas if the amount of this ingredient exceeds 18%, resistance todevitrification deteriorates. For the same reason as in the case ofSiO₂, a more preferable range of MgO is 13-18%. For the same reason asin the case of MgO, a preferable range of MgO+ZnO is 10-18% and a morepreferable range thereof is 13-18%.

[0137] The Al₂O₃ ingredient is a very important ingredient which, byheat treatment of the base glass, produces cordierite (Mg₂Al₄Si₅O₁₈),cordierite solid solution (Mg₂Al₄Si₅O₁₈ solid solution), spinel, spinelsolid solution and β-quartz solid solution (β-SiO₂ solid solution) aspredominant crystal phases. If the amount of this ingredient is lessthan 10%, the desired crystal phase cannot be obtained, the crystalphase of the obtained glass-ceramics is not stable and its texturebecomes coarse and, moreover, the melting property deteriorates. If theamount of this ingredient 20% or over, the melting property andresistance to devitrification deteriorate and an amount of precipitationof spinel increases abnormally with the result that hardness increasesexcessively and processability in polishing of members such as spacerrings deteriorates significantly and, moreover, specific gravityincreases to such a degree that the glass-ceramics are not suitable foruse in an information storage disk drive device in high speed rotation.Accordingly, a preferable range of Al₂O₃ is 10-less than 20%, a morepreferable range thereof is 10-18% and the most preferable range thereofis 12-18%.

[0138] The P₂O₅ ingredient functions as a nucleating agent for the glassand also is effective for improving the melting and forming propertiesand resistance to devitrification of the base glass. It will suffice ifan amount of 4% or less of this ingredient is added and a morepreferable range thereof is 1-3%.

[0139] The B₂O₃ ingredient is effective for controlling viscosity of thebase glass during melting and forming thereof. It will suffice if anamount of 4% or less of this ingredient is added.

[0140] The CaO ingredient is an ingredient which improves the meltingproperty of the glass and prevents the precipitating crystal frombecoming coarse. If the amount of this ingredient is less than 0.5%,these effects cannot be obtained whereas if the amount of thisingredient exceeds 4%, the precipitating crystal becomes coarse, thecrystal phase changes and chemical durability deteriorates. A morepreferable range thereof is 1-3%.

[0141] The SrO ingredient may be added for improving the meltingproperty of the glass. Addition of this ingredient in an amount notgreater than 2% will suffice. The BaO ingredient may also be added forimproving the melting property of the glass. Addition of this ingredientin an amount not greater than 5% will suffice. A more preferable rangethereof is 1-3%.

[0142] The ZrO₂ and TiO₂ ingredients are important ingredients whichfunction as a nucleating agent and, moreover, have been found to have aremarkable effect for making precipitating crystals fine and improvemechanical strength and chemical durability of the material. The ZrO₂ingredient in an amount not greater than 5% will suffice. If the amountof the TiO₂ ingredient is less than 2.5%, these effects cannot beobtained whereas if the amount of this ingredient exceeds 12%, meltingof the base glass becomes difficult and resistance to devitrificationdeteriorates. For the same reason as in the case of SiO₂, a morepreferable range of the total amount of ZrO₂ and TiO₂ is 2-12%.

[0143] The Bi₂O₂ ingredient is effective for restraining devitrificationwithout impairing the melting and forming properties of the base glass.Addition of this ingredient in an amount not greater than 6% willsuffice.

[0144] The Sb₂O₃ and As₂O₃ ingredients may be used as a refining agentin melting the glass but addition of an amount not greater than 1% ofeach ingredient will suffice.

[0145] The F ingredient may be added for improving the melting propertyof the glass but addition of an amount not greater than 3% will suffice.The Fe₂O₃ ingredient may be added as a coloring agent for the glass orimproving sensitivity of detection of a surface defect by utilizingcoloring of the glass but addition of an amount not greater than 5% willsuffice.

[0146] Description will now be made about suitable crystal phases andcompositions of the glass-ceramics which constitute the informationstorage disk holding members of the fourth aspect of the invention.

[0147] In the fourth aspect of the invention, the glass-ceramics whichconstitute the information storage disk holding members comprise, as apredominant crystal phase or phases, at least one crystal phase selectedfrom the group consisting of enstatite (MgSiO₃), enstatite solidsolution (MgSiO₃ solid solution), magnesium titanate (MgTi₂O) andmagnesium titanate solid solution ((MgTi₂O₅ solid solution). This isbecause these crystal phases are advantageous in that they contribute toincreasing rigidity and making crystal grain diameter of precipitatingcrystals relatively fine and, further have sufficient processability inpolishing.

[0148] For obtaining the above described desired physical properties,the glass-ceramics which contain, as a first phase which has the largestprecipitation ratio, enstatite (MgSiO₃), enstatite solid solution(MgSiO₃ solid solution), magnesium titanate (MgTi₂O₅) or magnesiumtitanate solid solution (MgTi₂O₅ solid solution) are particularlypreferable. As to crystal phases which have a smaller precipitationratio than the first phase, in case the first phase is enstatite(MgSiO₃) or enstatite solid solution (MgSiO₃ solid solution), it ispreferable for the glass-ceramics to contain at least one crystal phaseselected from the group consisting of magnesium titanate (MgTi₂O₅),magnesium titanate solid solution (MgTi₂O₅ solid solution), spinel andspinel solid solution. In case the first phase is magnesium titanate(MgTi₂O₅) or magnesium titanate solid solution (MgTi₂O₅ solid solution),it is preferable for the glass-ceramics to contain at least one crystalphase selected from the group consisting of enstatite (MgSiO₃),enstatite solid solution ((MgSiO₃ solid solution), spinel and spinelsolid solution.

[0149] In the fourth aspect of the invention, it is particularlypreferable for the glass-ceramics constituting the information storagedisk holding member to have enstatite (MgSiO₃) or enstatite solidsolution (MgSiO₃ solid solution) as a crystal phase having the largestprecipitation ratio (first phase).

[0150] In the fourth aspect of the invention, it is preferable for therespective predominant crystal phases to have a crystal grain diameterwithin a range of 0.05 μm-0.30 μm.

[0151] Reasons for preferable compositions of the glass-ceramics whichconstitute the information storage disk holding members of the fourthaspect of the invention will be described.

[0152] The SiO₂ ingredient is a very important ingredient which, by heattreatment of the base glass, produces enstatite (MgSiO₃) or enstatitesolid solution (MgSiO₃ solid solution) as predominant crystal phases. Ifthe amount of the SiO₂ ingredient is less than 40%, the crystal phase ofthe obtained glass-ceramics is not stable and its texture becomes coarseand, moreover, resistance to devitrification deteriorates whereas if theamount of this ingredient exceeds 60%, melting and forming of the baseglass become difficult.

[0153] The MgO ingredient is a very important ingredient which, by heattreatment of the base glass, produces enstatite (MgSiO₃), enstatitesolid solution (MgSiO₃ solid solution), magnesium titanate (MgTi₂O₃),magnesium titanate solid solution (MgTi₂O₅ solid solution), spinel andspinel solid solution as predominant crystal phases. If the amount ofthis ingredient is less than 10%, the desired crystal cannot be obtainedand, even if they are obtained, the crystal phase of the obtainedglass-ceramics is not stable and its texture becomes coarse and,moreover, the melting property deteriorates whereas if the amount ofthis ingredient exceeds 20%, resistance to devitrification deteriorates.

[0154] The Al₂O₃ ingredient is a very important ingredient which, byheat treatment of the base glass, produces enstatite solid solution(MgSiO₃ solid solution), magnesium titanate ((MgTi₂O₅) solid solution,spinel and spinel solid solution as predominant crystal phases. If theamount of this ingredient is less than 10%, the desired crystal phasecannot be obtained and, even if it is obtained, the precipitatingcrystal phase of the obtained glass-ceramics is not stable and itstexture becomes coarse and, moreover, the melting property deteriorates.If the amount of this ingredient is 20% or over, the melting propertyand resistance to devitrification of the base glass deteriorate and,moreover, spinel phase becomes predominant as the first phase with theresult that hardness increases excessively and processability inpolishing of members such as spacer rings deteriorates significantlywhich is undesirable from the standpoint of processability. Accordingly,a preferable range of A₂O is 10—less than 18% and a more preferablerange thereof is 10-17%.

[0155] The CaO ingredient is an ingredient which improves the meltingproperty of the glass and prevents the precipitating crystal frombecoming coarse. If the amount of this ingredient is less than 0.5%,these effects cannot be obtained whereas if the amount of thisingredient exceeds 4%, the precipitating crystal becomes coarse, thecrystal phase changes and chemical durability deteriorates.

[0156] The SrO ingredient may be added for improving the meltingproperty of the glass. If the amount of this ingredient is less than0.5%, this effect cannot be obtained. Addition of this ingredient in anamount not greater than 4% will suffice.

[0157] The BaO ingredient may also be added for improving the meltingproperty of the glass. Addition of this ingredient in an amount notgreater than 5% will suffice.

[0158] The ZrO₂ and TiO₂ ingredients are very important ingredientswhich function as a nucleating agent and, moreover, have been found tohave a remarkable effect for making precipitating crystals fine andimprove mechanical strength and chemical durability of the material. TheZrO₂ ingredient in an amount not greater than 5% will suffice. As toTiO₂ ingredient, if the amount of the TiO₂ ingredient is 8% or below,softening sometimes occurs during crystallization whereas if the amountof this ingredient exceeds 12%, melting of the base glass becomesdifficult and resistance to devitrification deteriorates.

[0159] The Bi₂O₂ ingredient is effective for restraining devitrificationwithout impairing the melting and forming properties of the base glass.If the amount of this ingredient exceeds 6%, corrosion of a materialsuch as Pt and SiO₂ which constitute a melting crucible becomessignificant.

[0160] The Sb₂O₃ and As₂O₃ ingredients may be used as a refining agentin melting the glass but addition of an amount not greater than 1% ofeach ingredient will suffice.

[0161] Within a scope not impairing the characteristics of theinvention, an optional element selected from P, W, Nb, La, Y and Pb maybe added in an amount of up to 3% on oxide basis and/or an optionalelement selected from Cu, Co, Fe, Mn, Cr, Sn and V may be added in anamount of up to 2% on oxide basis.

[0162] In the fourth aspect of the invention, it is preferable for theglass-ceramics to be substantially free of Li₂O, Na₂O and K₂O.

[0163] As the fifth aspect of the invention, preferable crystal phasesand compositions of the glass-ceramics which constitute the informationstorage disk holding members will now be described The glass-ceramicswhich constitutes the information storage disk holding member of thefifth aspect of the invention should preferably comprise at least onecrystal phase selected from the group consisting of β-quartz, β-quartzsolid solution, enstatite, enstatite solid solution, forsterite andforsterite solid solution as a predominant crystal phase or phases.

[0164] This is because these crystal phases are advantageous in thatthey contribute to increasing rigidity, making the glass-ceramicsrelatively of a low specific gravity and, moreover, making the graindiameter of the precipitating crystals very fine. In the fifth aspect ofthe invention, precipitation and ratios of precipitation of β-quartz,enstatite and forsterite in the glass-ceramics are determined by theratio of MgO and SiO₂ and precipitation and ratios of precipitation ofthese three crystal phases and solid solutions of these three crystalphases are determined by the ratios of the MgO and SiO₂ ingredients andother ingredients.

[0165] Reasons for defining compositions of the fifth aspect of theinvention will now be described.

[0166] The SiO₂ ingredient is a very important ingredient which, by heattreatment of the base glass, produces β-quartz, β-quartz solid solution,enstatite, enstatite solid solution, forsterite and forsterite solidsolution as predominant crystal phases. If the amount of the SiO₂ingredient is less than 40%, the crystal phase of the obtainedglass-ceramics is not stable and its texture becomes coarse whereas ifthe amount of this ingredient exceeds 60%, melting and forming of thebase glass become difficult. For precipitation of these crystal phases,conditions of heat treatment are also an important factor and a morepreferable range of this ingredient for enabling broader heat treatmentconditions is 48.5-58.5%.

[0167] The MgO ingredient is a very important ingredient which, by heattreatment of the base glass, produces β-quartz, β-quartz solid solution,enstatite, enstatite solid solution, forsterite and forsterite solidsolution as predominant crystal phases. If the amount of this ingredientis less than 10%, the crystal phase of the obtained glass-ceramics isnot stable and its texture becomes coarse and, moreover, the meltingproperty deteriorates whereas if the amount of this ingredient exceeds20%, resistance to devitrification and chemical durability of the baseglass deteriorates. For the same reason as in the case of SiO₂, a morepreferable range of MgO is 12-18%.

[0168] The Al₂O₃ ingredient is a very important ingredient which, byheat treatment of the base glass, produces β-quartz solid solution as apredominant crystal phase. If the amount of this ingredient is less than10%, the crystal phase of the obtained glass-ceramics is not stable andits texture becomes coarse whereas if the amount of this ingredientexceeds 20%, the melting property and resistance to devitrificationdeteriorate. For the same reason as described above, a more preferablerange thereof is 12-18%.

[0169] The P₂O₅ ingredient functions as a nucleating agent for the glassand also is effective for improving the melting property of the baseglass and improving resistance to devitrification during forming of theglass. If the amount of this ingredient is less than 0.5%, these effectscannot be obtained whereas if the amount of this ingredient exceeds2.5%, resistance to devitrification deteriorates. A preferable range ofthis ingredient is 1-2%.

[0170] The B₂O₃ ingredient is effective for controlling viscosity of thebase glass during melting and forming thereof. If the amount of thisingredient is less than 1%, these effects cannot be obtained whereas ifthe amount of this ingredient exceeds 4%, the melting property of thebase glass deteriorates and the precipitating crystal of theglass-ceramics are not stable and its texture becomes coarse. Apreferable range of this ingredient is 1-3%.

[0171] The Li₂O ingredient is a very important ingredient which, by heattreatment of the base glass, produces β-quartz solid solution as apredominant crystal phase and, moreover, improves the melting propertyof the base glass. If the amount of this ingredient is less than 0.5%,these effects cannot be obtained whereas if the amount of thisingredient exceeds 4%, the precipitating crystal of the obtainedglass-ceramics is not stable and its texture becomes coarse. Apreferable range of this ingredient is 1-3%.

[0172] The CaO ingredient is an ingredient which improves the meltingproperty of the glass and prevents the precipitating crystal frombecoming coarse. If the amount of this ingredient is less than 0.5%,these effects cannot be obtained whereas if the amount of thisingredient exceeds 4%, it becomes difficult to obtain the desiredcrystal and the precipitating crystal becomes coarse and chemicaldurability deteriorates. A more preferable range thereof is 1-3%.

[0173] The ZrO₂ and TiO₂ ingredients are important ingredients whichfunction as a nucleating agent and, moreover, have been found to have aremarkable effect for making precipitating crystals fine and improvemechanical strength and chemical durability of the material. If theamount of the ZrO₂ ingredient is less than 0.5% and the amount of theTiO₂ ingredient is less than 2.5%, these effects cannot be obtained. Ifthe amount of the ZrO₂ ingredient exceeds 5% and the amount of the TiO₂ingredient exceeds 8%, melting of the base glass becomes difficult,substance such as ZrSiO₄ which is left unmelted is produced andresistance to devitrification deteriorates with the result that abnormalgrowth of grain of crystal grains take place during crystallizationprocess. A preferable range of the total amount of ZrO₂ and TiO₂ is 9%or less. A preferable range of ZrO₂ is 1-4%, a preferable range of TiO₂is 3-7.5% and a more preferable range of the total amount of TiO₂ andZrO₂ is 3-8%.

[0174] The Sb₂O₃ and As₂O₃ ingredients may be added as a refining agentin melting the glass but addition of an amount not greater than 0.5% ofeach ingredient will suffice. As to the Sb₂O₃ ingredient, it ispreferable to add an amount not smaller than 0.01%.

[0175] The SnO₂, MoO₃, CeO and Fe₂O₃ ingredients may be added up to 5%for each of these ingredients for imparting color to the glass orimproving sensitivity of detection of surface defects by impartingcolor, and improving absorption characteristic of an LD excited solidlaser. The SnO₂, CeO and Fe₂O₃ ingredients should preferably be added inan amount not greater than 5% and the MoO₃ ingredient should preferablybe added in an amount not greater than 3%. The SnO₂ and MoO₃ ingredientsare important ingredients which have a light transmitting property inthe state of glass before heat treatment and are imparted with colorafter crystallization by the heat treatment.

[0176] Description will now be made about preferable crystal phases andcompositions which constitute the holding members of the sixth aspect ofthe invention.

[0177] In the sixth aspect of the invention, the glass-ceramics whichconstitute the information storage disk holding members shouldpreferably comprise at least one crystal phase selected from the groupconsisting of cordierite, cordierite solid solution, spinel, spinelsolid solution, enstatite, enstatite solid solution, β-quartz andβ-quartz solid solution.

[0178] This is because these crystal phases are advantageous in thatthey have excellent processability, contribute to increasing rigidity,making the grain diameter of the precipitating crystals relativelysmall, and making the glass-ceramics relatively of a low specificgravity. Precipitation and ratios of precipitation of cordierite,spinel, enstatite and β-quartz in the glass-ceramics are determined bythe ratio of MgO, SiO₂ and Al₂O₃. Precipitation and ratios ofprecipitation of these four crystal phases and solid solutions of thesefour crystal phases are determined by the ratios of the MgO, SiO₂ andAl₂O₃, ingredients and other ingredients. In the sixth aspect of theinvention, the crystal grain diameter of the respective predominantcrystal phases of the glass-ceramics should preferably be within a rangeof 0.05 μm-0.30 μm.

[0179] The SiO₂ ingredient is a very important ingredient which, by heattreatment of the base glass, produces cordierite, cordierite solidsolution, enstatite, enstatite solid solution, β-quartz and β-quartzsolid solution as predominant crystal phases. If the amount of the SiO₂ingredient is less than 40%, the crystal phase of the obtainedglass-ceramics is not stable and its texture becomes coarse whereas ifthe amount of this ingredient exceeds 60%, melting and forming of thebase glass become difficult. For precipitation of these crystal phases,conditions of heat treatment are also an important factor and a morepreferable range of this ingredient for enabling broader heat treatmentconditions is 48.5-58.5%.

[0180] The MgO ingredient is a very important ingredient which, by heattreatment of the base glass, produces cordierite, cordierite solidsolution, spinel, spinel solid solution, enstatite, enstatite solidsolution, β-quartz and β-quartz solid solution as predominant crystalphases. If the amount of this ingredient is less than 10%, the desiredcrystal cannot be obtained, the crystal phase of the obtainedglass-ceramics is not stable and its texture becomes coarse and,moreover, the melting property deteriorates whereas if the amount ofthis ingredient exceeds 20%, resistance to devitrification deteriorates.For the same reason as in the case of SiO₂, a more preferable range ofMgO is 13-20%.

[0181] The Al₂O₃ ingredient is a very important ingredient which, byheat treatment of the base glass, produces cordierite, cordierite solidsolution, spinel, spinel solid solution and β-quartz solid solution aspredominant crystal phases. If the amount of this ingredient is lessthan 10%, the desired crystal phase cannot be obtained, the crystalphase of the obtained glass-ceramics is not stable and its texturebecomes coarse and, moreover, the melting property deteriorates. If theamount of this ingredient exceeds 20%, the melting property andresistance to devitrification deteriorate and an amount of precipitationof spinel increases abnormally with the result that hardness increasesexcessively and processability in polishing deteriorates significantly.For the same reason as described above, a preferable range of Al₂O₃ is10-less than 18% and a more preferable range thereof is 10-17%.

[0182] The P₂O₅ ingredient functions as a nucleating agent for the glassand also is effective for improving the melting and forming propertiesand resistance to devitrification of the base glass. It will suffice ifan amount of 4% or less of this ingredient is added and a morepreferable range thereof is 1-3%.

[0183] The B₂O₃ ingredient is effective for controlling viscosity of thebase glass during melting and forming thereof. It will suffice if anamount of 4% or less of this ingredient is added.

[0184] The CaO ingredient is an ingredient which improves the meltingproperty of the glass and prevents the precipitating crystal frombecoming coarse. If the amount of this ingredient is less than 0.5%,these effects cannot be obtained whereas if the amount of thisingredient exceeds 4%, the precipitating crystal becomes coarse, thecrystal phase changes and chemical durability deteriorates. A morepreferable range thereof is 1-3%.

[0185] The BaO ingredient may be added for improving the meltingproperty of the glass. Addition of this ingredient in an amount notgreater than 5% will suffice. A more preferable range thereof is 1-3%.

[0186] The ZrO₂ and TiO₂ ingredients are important ingredients whichfunction as a nucleating agent and, moreover, have been found to have aremarkable effect for making precipitating crystals fine and improvemechanical strength and chemical durability of the material. The ZrO₂ingredient in an amount not greater than 5% will suffice. If the amountof the TiO₂ ingredient is less than 2.5%, these effects cannot beobtained whereas if the amount of this ingredient exceeds 8%, melting ofthe base glass becomes difficult and resistance to devitrificationdeteriorates. For the same reason as in the case of SiO₂, a morepreferable range is 2-8%.

[0187] The Sb₂O₃ and As₂O₃ ingredients may be used as a refining agentin melting the glass but addition of an amount not greater than 1% ofeach ingredient will suffice.

[0188] The F ingredient may be added for improving the melting propertyof the glass. Addition of this ingredient in an amount not greater than3% will suffice.

[0189] The Fe₂O₃ ingredient may be added as a coloring agent for theglass or improving sensitivity of detection of a surface defect byutilizing coloring of the glass and improving laser absorptioncharacteristic of an LD excited laser but addition of this ingredient inan amount not greater than 5% will suffice.

[0190] Description will now be made about preferable crystal phases andcompositions of the glass-ceramics for the holding members of theseventh aspect of the invention.

[0191] In the seventh aspect of the invention, the information storagedisk holding member is made of the glass-ceramics which comprise, as apredominant crystal phase or phases, at least one crystal phase selectedfrom the group consisting of β-quartz (β-SiO₂), β-quartz solid solution(β-SiO₂ solid solution), β-spodumene (β-Li₂O.Al₂O₃.SiO₂), β-spodumenesolid solution (β-Li₂O.Al₂O₃.SiO₂ solid solution), β-eucryptite(β-Li₂O.Al₂O₃.2SiO₂ where a part of Li₂O is replaceable by MgO and/orZnO) and eucryptite solid solution (β-Li₂O.Al₂O₃.2SiO₂ solid solutionwhere a part of Li₂O is replaceable by MgO and/or ZnO).

[0192] Precipitation and ratios of precipitation of one or more crystalphases selected from β-quartz, β-spodumene and β-eucryptite aredetermined by ratio of amount of Li₂O to amounts of Al₂O₃ and SiO₂within the specific composition ranges. Precipitation and ratios ofprecipitation of one or more crystal phases selected from these crystalphases and solid solution phases of the respective crystal phases aredetermined by amounts of other ingredients.

[0193] Description will be made about respective ingredients.

[0194] In the seventh aspect of the invention, the SiO₂ ingredient inthe glass-ceramics which constitute the information storage disk holdingmember is a very important ingredient which, by heat treatment of thebase glass, produces the above described crystals as predominant crystalphases. If the amount of this ingredient is less than 50%, theprecipitating crystal of the obtained glass-ceramics is not stable andits texture tends to become coarse with resulting deterioration inmechanical strength and increase in surface roughness obtained bypolishing. If the amount of this ingredient exceeds 62%, melting andforming of the base glass become difficult and homogeneity isdeteriorated. A preferable range of this ingredient is 53-57% and a morepreferable range thereof is 54-56%.

[0195] The P₂O₅ ingredient is effective for improving melting andclarity of the base glass by coexistence with the SiO₂ ingredient. Ifthe amount of this ingredient is less than 5%, this effect cannot beobtained whereas if the amount of this ingredient exceeds 10%,resistance to devitrification of the base glass deteriorates with theresult that texture of the glass-ceramics becomes coarse in thecrystallization process and mechanical strength thereby is reduced. Apreferable range of this ingredient is 6-10% and a more preferable rangethereof is 7-9%.

[0196] For attaining this effect significantly, a preferable range ofSiO₂+P₂O₅ is 61-65% and a preferable range of P₂O₅/SiO₂ is 0.12-0.16. Amore preferable range of SiO₂+P₂O₅ is 62-64% and a preferable range ofP₂O₅/SiO₂ is 0.13-0.15.

[0197] If the amount of the Al₂O₃ ingredient is less than 22%, meltingof the base glass becomes difficult and, therefore, homogeneity of theglass-ceramics obtained deteriorates and chemical durability of the baseglass-ceramics also deteriorates. If the amount of this ingredientexceeds 26%, the melting property of the base glass deteriorates withresulting deterioration in homogeneity and, moreover, resistance todevitrification of the base glass deteriorates with the result that thetexture of the glass-ceramics become coarse and mechanical strengththereby is reduced. A preferable range of this amount is 23-26% and amore preferable range thereof is 23-25%.

[0198] The three ingredients of Li₂O, MgO and ZnO are importantingredients which constitute β-quartz solid solution, β-spodumene,β-spodumene solid solution, β-eucryptite and β-eucryptite solidsolution. These three ingredients are also important in that, bycoexistence with the above described limited ranges of SiO₂ and P₂O₅ingredients, improve the low expansion characteristic of theglass-ceramics, reduce an amount of deflection in high temperatures and,further, improve the melting property and clarity of the base glasssignificantly.

[0199] If the amount of the Li₂O ingredient is less than 3%, theseeffects cannot be obtained and, moreover, decrease in homogeneity due todeterioration in the melting property takes place and difficulty arisesin precipitation of the desired crystal phase. If the amount of thisingredient exceeds 5%, the low expansion characteristic cannot beobtained and resistance to devitrification deteriorates with the resultthat the texture of the glass-ceramics becomes coarse in thecrystallization process and mechanical strength thereby is reduced. Apreferable range of this ingredient is 3.5-5% and a more preferablerange thereof is 3.5-4.5%.

[0200] If the amount of the MgO ingredient is less than 0.5%, the abovedescribed effect cannot be obtained whereas if the amount of thisingredient exceeds 2%, the low expansion characteristic cannot beobtained A preferable range of this ingredient is 0.5-1.8% and a morepreferable range thereof is 0.6-1.5%.

[0201] If the amount of the ZnO ingredient is less than 0.2%, the abovedescribed effect cannot be obtained whereas if the amount of thisingredient exceeds 2%, the low expansion characteristic cannot beobtained and resistance to devitrification deteriorates with the resultthat the texture of the glass-ceramics becomes coarse in thecrystallization process and mechanical strength thereby is reduced. Apreferable range of this ingredient is 0.2-1.8% and a more preferablerange thereof is 0.2-1.5%.

[0202] For attaining the above described effects significantly, thetotal amount of the three ingredients of Li₂O, MgO and ZnO should bewithin a range of 4.0-6.5%. A more preferable range thereof is 4.3-6.5%and a more preferable range thereof is 4.5-6.5%.

[0203] The two ingredients of CaO and BaO basically remain as a glassmatrix other than the crystals which have precipitated in the glass andare important in that they function to perform fine adjustment betweenthe crystal phases and the glass matrix phase with respect to the abovedescribed low expansion characteristic and improvement in the meltingproperty.

[0204] If the amount of the CaO ingredient is less than 0.3%, thiseffect cannot be obtained whereas if the amount of this ingredientexceeds 4%, the desired crystal phase cannot be obtained and, moreover,resistance to devitrification deteriorates with the result that thetexture of the glass-ceramics becomes coarse in the crystallizationprocess and mechanical strength thereby is reduced. A preferable rangeof this ingredient is 0.5-3% and a more preferable range thereof is0.5-2%.

[0205] If the amount of the BaO ingredient is less than 0.5%, thiseffect cannot be obtained whereas if the amount of this ingredientexceeds 4%, resistance to devitrification deteriorates with the resultthat the texture of the glass-ceramics becomes coarse in thecrystallization process and mechanical strength thereby is reduced. Apreferable range of this ingredient is 0.5-3% and a more preferablerange thereof is 0.5-2%.

[0206] For attaining the above described effect significantly, the totalamount of CaO and BaO should be within a range of 0.8-5%. A preferablerange thereof is 1-4% and a more preferable range thereof is 1-3%.

[0207] The TiO₂ and ZrO₂ ingredients are indispensable as a nucleatingagent. If the amount of each of these ingredients is less than 1%, adesired crystal cannot precipitate whereas if the amount of each ofthese ingredients exceeds 4%, the melting property of the base glassdeteriorates with resulting deterioration in homogeneity and, in theworst case, an unmelted substance remains in the glass. A preferablerange of TiO₂ is 1.5-4% and a preferable range of ZrO₂ is 1.5-3.5%. Amore preferable range of TiO₂ is 1.5-3.5% and a more preferable range ofZrO₂ is 1-3%.

[0208] The As₂O₃ and Sb₂O₃ ingredients may be added as a refining agentin melting of the glass for obtaining a homogeneous product. Addition ofthese ingredients in an amount not greater than 4% will suffice. Apreferable range of As₂O₃+Sb₂O₃ is 0-2% and a more preferable range is0-2% of As₂O_(3.)

[0209] In addition to the above described ingredients, for fineadjustment of properties and other purposes and within the scope notimpairing the characteristics of the information storage disk holdingmember of the invention, one or more of SrO, B₂O₃, F₂, La₂O₃, Bi₂O₃,WO₃, Y₂O₃, Gd₂O₃ and SnO₂ may be added in the total amount of notgreater than 2%. In addition, one or more coloring agents such as CoO,NiO, MO₂, Fe₂O₃ and Cr₂O₃ may be added in the total amount of notgreater than 2%.

[0210] Description will be made about preferable crystal phases andcompositions of the holding member of the eighth aspect of theinvention.

[0211] The information storage disk holding members of the eighth aspectof the invention can be made of glass-ceramics which comprise gahnite(ZnAl₂O₃) and/or gahnite solid solution (ZnAl₂O₃ solid solution) aspredominant crystal phase or phases.

[0212] In the eighth aspect of the invention, precipitation and ratio ofprecipitation of gahnite and/or gahnite solid solution are determined byratios of amounts of ingredients other than ZnO and Al₂O₃ whichconstitute gahnite.

[0213] The precipitating predominant crystal phase is an importantfactor which influences the coefficient of thermal expansion. It ispreferable to cause a crystal phase having a negative coefficient ofthermal expansion to precipitate against glass which has a positivecoefficient of thermal expansion and thereby realize a coefficient ofthermal expansion of the glass-ceramics as a whole within a desiredrange.

[0214] In the glass-ceramics constituting the information storage diskholding members of the eighth aspect of the invention, if the amount ofthe SiO₂ ingredient is less than 30%, the crystal grain tends to becomecoarse and chemical durability and mechanical strength deterioratewhereas if the amount of this ingredient exceeds 65%, melting of thebase glass becomes difficult and homogeneity thereby deteriorates. Apreferable range of this ingredient is 32-63% and a more preferablerange thereof is 34-61%.

[0215] If the amount of the Al₂O₃ ingredient is less than 5%,precipitation of gahnite as a predominant crystal phase becomesdifficult whereas if the amount of this ingredient exceeds 35%, themelting property deteriorates resulting in deterioration of homogeneityand resistance to devitrification of the base glass deteriorates withthe result that the texture of the glass-ceramics becomes coarse in thecrystallization process and mechanical strength thereby is reduced. Apreferable range of this ingredient is 7-33% and a more preferable rangethereof is 10-30%.

[0216] The ZnO ingredient is an important ingredient which, by heattreatment of the base glass, produces gahnite as a predominant crystalphase with the Al₂O₃ ingredient and improves mechanical strength andheat resisting property of the substrate. If the amount of thisingredient is less than 5%, this effect cannot be obtained whereas ifthe amount of this ingredient exceeds 35%, resistance to devitrificationdeteriorates with the result that the texture of the glass-ceramicsbecomes coarse in the crystallization process and mechanical strengththereby is reduced. A preferable range of this ingredient is 7-33% and amore preferable range thereof is 10-30%.

[0217] If the amount of the MgO ingredient is less than 1%, the meltingproperty deteriorates and thereby homogeneity deteriorates and,moreover, resistance to devitrification deteriorates with the resultthat the texture of the glass-ceramics becomes coarse in thecrystallization process and mechanical strength thereby is reduced. Ifthe amount of this ingredient exceeds 20%, resistance to devitrificationof the base glass deteriorates. A preferable range of this ingredient is3-18% and a more preferable range thereof is 3-15%.

[0218] The CaO, SrO, BaO, B₂O₃, La₂O₃, Y₂O₃, Gd₂O₃, Ta₂O₃, Nb₂O₅, WO₃and Bi₂O₃ingredients are effective for improving the melting property ofthe base glass and the La₂O₃, Y₂O₃, Gd₂O₃, Ta₂O₃, Nb₂O₅, WO₃, andBi₂O₃ingredients are effective also for improving mechanical strengthand chemical durability of the product. For obtaining these effectswhile preventing the crystal phase which precipitates by heat treatmentfrom becoming coarse, the total amount of one or more of theseingredients should be within a range of 0.5%-20%. If, however, theamount of B₂O₃ exceeds 10% or the total amount of one or more of theTa₂O₃, Nb₂O₅, WO₃ and Bi₂O₃ingredients exceeds 10%, difficulty arises inprecipitation of a desired crystal phase. A preferable range ofCaO+SrO+BaO+B₂O₃+La₂O₃+Y₂O₃+Gd₂O₃+Ta₂O₃+Nb₂O₅+WO₃+Bi₂O₃ is 0.5-15%. Apreferable range of B₂O₃ is 0-8%. A preferable range ofTa₂O₃+Nb₂O₅+WO₃+Bi₂O₃ is 0-5%. A more preferable range ofCaO+SrO+BaO+B₂O₃+La₂O₃+Y₂O₃+Gd₂O₃+Ta₂O₃+Nb₂O₅+WO₃+Bi₂O₃ is 0.5-10%. Amore preferable range of B₂O₃ is 0-5%. A more preferable range ofTa₂O₃+Nb₂O₅+WO₃+Bi₂O₃ is 0-5%.

[0219] The TiO₂ ingredient is indispensable as a nucleating agent. Ifthe amount of this ingredient is less than 1%, a desired crystal phasecannot be produced whereas if the amount of this ingredient exceeds 15%,resistance to devitrification deteriorates with the result that thetexture of the glass-ceramics becomes coarse in the crystallizationprocess and mechanical strength thereby is reduced. A preferable rangeof this ingredient is 3-13% and a more preferable range thereof is4-10%.

[0220] The ZrO₂, P₂O₅ and SnO₂ ingredients may be employed as anauxiliary nucleating agent. The total amount of these ingredients shouldpreferably be not greater than 7%. The amount of the ZrO₂ ingredientshould preferably be less than 2%, the amount of the P₂O₅ ingredientshould preferably be not greater than 5% and the amount of the SnO₂ingredient should preferably be not greater than 2%. If the amounts ofthese ingredients exceed the above ranges, resistance to devitrificationdeteriorates with the result that the texture of the glass-ceramicsbecomes coarse in the crystallization process and mechanical strengththereby is reduced. More preferably, the amount of ZrO₂+P₂O₅+SnO₂ shouldnot be greater than 6%, the amount of ZrO₂ should be less than 1.8%, theamount of P₂O₅ should not be greater than 4.5% and the amount of SnO₂should not be greater than 1.8%. Most preferably, the amount ofZrO₂+P₂O₅+SnO₂ should not be greater than 5%, the amount of ZrO₂ shouldbe less than 1.7%, the amount of P₂O₅ should not be greater than 4% andthe amount of SnO₂ should not be greater than 1.7%.

[0221] The As₂O₃ and Sb₂O₃ ingredients may be added as a refining agentin melting of the glass. Addition of these ingredients in the totalamount not greater than 4% will suffice. A preferable total amount is 3%or below and a more preferable total amount is 2% or below.

[0222] If one or more fluorides of the above described ingredients arecontained, it is effective as a flux of the base glass and is effectivefor adjusting crystallization. If the amount of the fluoride as thetotal amount of F exceeds 5%, the tendency to devitrification increasesto such a degree that a desired product cannot be obtained.

[0223] In addition to these ingredients, within a scope not impairingthe desired characteristics, coloring agents including MnO₂, NiO, CoO,Fe₂O₃, Cr₂O₃, V₂O₃, MoO₂ and Cu₂O, GeO₂ and one or more rare earthoxides other than those described above may be added in the total amountup to 10%.

[0224] Description will be made about the ninth aspect of the invention.For achieving the desired coefficient of thermal expansion, theglass-ceramics should preferably contain at least one crystal phaseselected from α-cristobalite, α-cristobalite solid solution, α-quartzand α-quartz solid solution which has a relatively large positivecoefficient of thermal expansion as a predominant crystal phase orphases. Particularly, by selecting these predominant crystal phases,glass-ceramics having excellent chemical durability and physicalproperties can be obtained easily. The grain diameter (average) of thesecrystal phases should preferably be less than 0.10 μm. Ratio of X-rayintensity of crystal phases other than these predominant crystal phasesshould preferably be less than 20, more preferably less than 10 and mostpreferably less than 5.

[0225] In the ninth aspect of the invention, the glass-ceramics shouldpreferably not contain lithium disilicate as a predominant crystal phasefrom the standpoint of mechanochemical influence in the polishingprocess. For adjusting the thermal expansion characteristic, theglass-ceramics should preferably not contain β-spodumene, β-eucryptiteor β-cristobalite which has a negative coefficient of thermal expansionand lithium silicate (Li₂O.SiO₂), diopside, enstatite, mica, α-tridymiteor fluorrichterite.

[0226] Description will now be made about the glass-ceramics whichconstitute the information storage disk holding members of the ninthaspect of the invention.

[0227] The SiO₂ ingredient is a very important ingredient which, by heattreatment of the base glass, produces α-cristobalite, α-cristobalitesolid solution, α-quartz and α-quartz solid solution as predominantcrystal phases. If the amount of this ingredient is less than 65%,precipitating crystals of the glass-ceramics obtained is not stable andits texture tends to become coarse whereas if the amount of thisingredient exceeds 75%, melting and forming of the base glass becomedifficult. A preferable range of this ingredient is a value exceeding65% and/or up to 75% and a more preferable range thereof is 68-74%.

[0228] The Li₂O ingredient is an important ingredient which improves themelting property of the glass. If the amount of this ingredient is lessthan 4%, this effect cannot be obtained and difficulty arises in meltingof the base glass. If the amount of this ingredient exceeds 7%, theproblem of dissolving out of Li ion arises and production of lithiumdisilicate crystal increases. A preferable range of this ingredient is4.5-6.5% and a more preferable range thereof is 4.5-6.0%.

[0229] The MgO, ZnO, SrO, BaO and CaO ingredients improve the meltingproperty of the glass and prevent the precipitating crystal frombecoming coarse. The total amount of these ingredients should preferablebe not smaller than 2%. If the total amount of these ingredients exceeds15%, crystals obtained become instable and their texture tends to becomecoarse.

[0230] In the present invention, the P₂O₅ ingredient is indispensable asa nucleating agent and, for enhancing forming of crystal nucleuses andpreventing a predominant crystal phase from becoming coarse, the amountof this ingredient should preferably be not smaller than 1.0%. Forpreventing the base glass from becoming opaque and maintaining stabilityin a large scale production, the amount of this ingredient shouldpreferably be not greater than 2.5%.

[0231] The ZrO₂ ingredient is an important ingredient which, like theP₂O₅ ingredient, functions as a nucleating agent and, moreover, has beenfound to have a remarkable effect for making precipitating crystals fineand improves mechanical strength and chemical durability of thematerial. The amount of the ZrO₂ ingredient should preferably be notsmaller than 2.0%. If an excessive amount of this ingredient is added,melting of the base glass becomes difficult and substance such as ZrSiO₄which is left unmelted is produced and, therefore, the amount of thisingredient should preferably be not greater than 7%. A more preferablerange thereof is 2-6% and, more preferably, the upper limit of thisingredient should be 5%.

[0232] The SnO₂ ingredient functions, like ZrO₂, as a nucleating agent.Addition of this ingredient in an amount not greater than 3% willsuffice.

[0233] The Al₂O₃ ingredient improves chemical durability and mechanicalstrength, particularly hardness, of the glass-ceramics and the amount ofthis ingredient should preferably be not smaller than 5%. If the amountof this ingredient is excessive, the melting property and resistance todevitrification deteriorate and the predominant crystal phase change toβ-spodumene (Li₂O.Al₂O₃.4SiO₂) which is a low expansion crystal.Precipitation of β-spodumene (Li₂O.Al₂O₃.4SiO₂) deteriorates thecoefficient of thermal expansion significantly and, therefore,precipitation of this ingredient should be avoided. For this reason, theamount of this ingredient should preferably be not greater than 9%. Morepreferably, the lower limit of this ingredient should not be smallerthan 5% and the upper limit thereof should be less than 9% and, mostpreferably, the lower limit should not be smaller than 6% and the upperlimit thereof should be less than 8%.

[0234] The Y₂O₃, WO₃, La₂O₃ and Bi₂O₃ ingredients are ingredients whichimprove the melting property which is reduced in a compositioncomprising a low content of Li₂O and also increase Young's modulus ofthe glass. Addition of these ingredients in the total amount of notgreater than 3% will suffice. If the total amount of these ingredientsexceeds 3%, precipitation of a stable crystal becomes difficult.

[0235] The Sb₂O₃ and As₂O₃ ingredients may be added as a refining agentin melting of he glass. Addition of these ingredients in the totalamount not greater than 1% will suffice.

[0236] Within a scope not impairing the above described characteristics,the Ga, Ge, Cu, Fe, Co, Nb, Ti, V, Ce, Gd and B ingredients may be addedin an amount up to 3% (mass % on oxide basis) for each ingredient. TheMo, Ta, Mn, Cr and F ingredients should preferably not be added ifpossible.

[0237] A preferable range of each of the Na₂O and K₂O ingredients is0-less than 3%. A preferable range of Na₂O+K₂O is 0.1-2.5%. The K₂O andNa₂O ingredients are effective for reducing the melting temperature andalso are effective for restraining dissolving out of alkali ion from theglass matrix by mixing with the Li₂O ingredient. This is because mixingand coexistence of small amounts of these alkali ingredients improveelectrical nature (volume resistivity). By mixing and coexistence of theK₂O and Na₂O ingredients in glass containing a relatively large amountof the Li₂O ingredient, volume resistivity is improved and mobility ofalkali ion in the glass thereby is restrained with the result thatdissolving out of alkali ion can be restrained.

[0238] In the first to ninth aspects of the invention, theglass-ceramics which constitute the information storage disk holdingmembers should be substantially free of the PbO ingredient which is notdesirable for environment.

[0239] In the information storage disk holding members made of theglass-ceramics of the invention, the total amount of the above describedingredients should not be smaller than 90% and, more preferably, be notsmaller than 95% and most preferably be not smaller than 98% forobtaining better information storage disk holding members.

[0240] The glass-ceramics which constitute the information storage diskholding members of the invention can be manufactured by melting glasshaving the above described compositions, subjecting the glass to hotforming or cold processing and thereafter subjecting the glass to heattreatment for nucleation and further heat treatment at a highertemperature for crystallization. By these treatments, the glass-ceramicswhich constitute the information storage disk holding members of theinvention have a structure in which crystal phases are dispersed inglass matrix and which does not substantially contain pores.

[0241] In the first and third aspects of the invention, theglass-ceramics which constitute the information storage disk holdingmembers of the invention can be manufactured by melting the glass havingthe above described compositions at a temperature within a range ofabout 1350° C.-1500° C., subjecting the glass to hot forming or coldprocessing, and thereafter subjecting the glass to heat treatment fornucleation at a temperature within a range of 450° C.-850° C. for 1 to12 hours, more preferably for 1 to 7 hours and further heat treatmentfor crystallization at a temperature within a range of 700° C.-1000° C.

[0242] In the method of manufacturing of the glass-ceramics whichconstitute the information storage disk holding members of the secondaspect of the invention, the melting temperature range of the glassshould preferably be about 1350° C.-1450° C., the range of nucleationtemperature should preferably be 450° C.-650° C., more preferably 500°C.-600° C. and the range of the crystallization temperature shouldpreferably be 700° C.-800° C., more preferably 720° C.-780° C. In thethird aspect of the invention, the range of the melting temperatureshould preferably be 1400° C.-1500° C., the range of nucleationtemperature should preferably be 600° C.-800° C., more preferably 650°C.-750° C. and the range of the crystallization temperature shouldpreferably be 800° C.-1000° C., more preferably 830° C.-980° C.

[0243] In the fourth aspect of the invention, the glass-ceramics whichconstitute the information storage disk holding members of the inventioncan be manufactured by subjecting the glass to heat treatment fornucleation at a temperature within a range of 400° C.-600° C. for 1 to 7hours and further heat treatment for crystallization at a temperaturewithin a range of 700° C. -760° C. for about 1-7 hours. By these heattreatment conditions, information storage disk holding members made ofglass-ceramics which have desired crystal phases, crystal graindiameter, amount of crystals and degree of crystallization can beobtained.

[0244] In the sixth aspect of the invention, the glass-ceramics whichconstitute the information storage disk holding members of the inventioncan be manufactured by subjecting the glass to heat treatment fornucleation at a temperature within a range of 650° C.-750° C. for 1 to12 hours and further heat treatment for crystallization at a temperaturewithin a range of 7500° C.-1050° C. for about 1-12 hours.

[0245] In the seventh and eighth aspects of the invention, theglass-ceramics which constitute the information storage disk holdingmembers of the invention can be manufactured by melting the glass havingthe above described compositions, subjecting the glass to hot forming orcold processing, and thereafter subjecting the glass to heat treatmentfor nucleation at a temperature within a range of 650° C.-750° C. for 1to 12 hours and further heat treatment for crystallization at atemperature within a range of 750° C.-950° C. for about 1 to 12 hours.

[0246] In the ninth aspect of the invention, the glass-ceramics whichconstitute the information storage disk holding members of the inventioncan be manufactured by mixing raw materials including oxides, carbonatesand nitrates having the above described composition, melting the rawmaterials at a temperature of about 1350° C.-1450° C. by using aconventional melting device, stirring and homogenizing the glass,forming the glass to the form of a disk and cooling it to obtain aformed glass, and thereafter subjecting the formed glass to heattreatment for nucleation at a temperature within a range of 400° C.-600°C. for about 1 to 7 hours and further heat treatment for crystallizationat a temperature within a range of 650° C.-750° C. for about 1 to 7hours.

[0247] For releasing static electricity stored in an information storagedisk, it is preferable to form an electrically conductive film on thesurface of the information storage disk holding members. The conductivefilm may be formed by a surface treatment using a hard metal or by flamespray of ceramics.

[0248] The spacer rings for the information storage disk of theinvention can be manufactured by pressing the glass-ceramics to a ringform in the hot forming or cold processing in the above describedmanufacturing process and lapping and polishing the surface of thespacer rings which comes into contact with the information storage disk.

[0249] By placing one or more information storage disks on a rotor hubthrough the spacer rings for the information storage disk, aninformation storage disk drive device suitable for high speedinformation transmission can be realized. In this case, it is preferablethat coefficients of thermal expansion of the rotor hub and spacer ringsshould be about equal to each other.

BRIEF DESCRIPTION OF THE DRAWINGS

[0250]FIG. 1 is a graph showing relationship between crystallizationtemperature and bending strength of Examples 1-1 to 1-3 andglass-ceramics of the same compositions.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0251] Preferred Examples of information storage disk holding members ofthe invention will now be described.

[0252] Average crystal grain diameters of respective crystal phases weremeasured by a transmission electron microscope (TEM). Types of crystalgrains of the respective crystal phases were identified by TEM structureanalysis or X-ray diffraction analysis (XRD). Description of the crystalphases is made in the order of a larger precipitation ratio. The orderof the precipitation ratio was determined in the order of height of themain peak of respective predominant crystal phases by XRD.

[0253] Surface roughness Ra (arithmetic mean roughness) was measured byan atomic force microscope (AFM).

[0254] Measurement of Li ion dissolving amount was conducted by ionchromatography. In the measurement, 80 ml (room temperature) of superpure water and a disk (having a diameter of 65 mm and thickness of 0.635mm) were packed in a film pack which was thereafter held in a drier withits temperature maintained at about 30° C. for 3 hours and then the diskwas taken out and the ion chromatography was conducted.

[0255] Average coefficient of linear expansion was measured by JOGIS(Japan Optical Glass Industry Standard) 06. Young's modulus was measuredby the ultrasonic pulse method of JIS R1602. Bending strength wasmeasured by JIS R1601 (three-point bending strength). Vickers' hardnesswas measured by JIS R1610. Transmittance of material having a platethickness of 10 mm at wavelength 950-1600 nm was measured by aspectrophotometer. Specific gravity was measured by JOGIS (Japan OpticalGlass Industry Standard) 06.

[0256] Examples and comparative examples of the information storage diskholding members were finished by lapping glass-ceramics of therespective examples and comparative examples with diamond pellets of800#-2000# for about 5 minutes to 30 minutes and then polishing themwith abrasive (cerium oxide) having a grain diameter (average) of 0.02μm to 3.0 μm for about 30 minutes to 60 minutes.

[0257] Preferred examples of the information storage disk holdingmembers of the first and third aspects of the invention will bedescribed. Tables 1-9 show Examples (1-1 to 1-14 and 3-1 to 3-8) ofspacer rings made of glass-ceramics and Comparative Examples (1 to 3) ofthe prior art glass-ceramics with respect to their compositions,nucleation and crystallization temperatures, crystal phase, averagecrystal grain diameter and amount of crystal of the crystal phase of theglass-ceramics, and degree of crystallization, coefficient of thermalexpansion, Young's modulus, specific gravity, specific rigidity (Young'smodulus/specific gravity) and bending strength of the glass-ceramics. Inexpressing crystal phases in these tables, solid solutions of therespective crystals are expressed by affixing “SS” after the names ofthe crystal phases. The invention is not limited to these examples.

[0258] In FIG. 1, relationship between crystallization temperature andbending strength of glass-ceramics having the same composition as thoseof Examples 1-1 to 1-3 is shown. TABLE 1 Examples 1-1 1-2 1-3 SiO₂ 75.375.3 75.3 Li₂O 9.9 9.9 9.9 P₂O₅ 2.0 2.0 2.0 ZrO₂ 2.3 2.3 2.3 Al₂O₃ 7.07.0 7.0 MgO 0.8 0.8 0.8 ZnO 0.5 0.5 0.5 KaO 2.0 2.0 2.0 Sb₂O₃ 0.2 0.20.2 Nucleation temperature (° C.) 540 540 540 Crystallizationtemperature (° C.) 720 740 760 Predominant crystal phase lithium lithiumlithium disilicate disilicate disilicate Li₂Si₂O₅ Li₂Si₂O₅ Li₂Si₂O₅Average grain diameter (μm) 0.005 μm 0.005 μm 0.020 μm Amount of crystal(mass %) 8 mass % 10 mass % 18 mass % Predominant crystal phase α-quartzα-quartz α-SiO₂ α-SiO₂ Average grain diameter (μm) 0.010 μm 0.030 μmDegree of crystallization (mass %) 8 26 43 Coefficient of thermalexpansion 65 74 110 (× 10⁻⁷/° C.) (−50° C.-+70° C.) Young's modulus(GPa) 108 98 95 Specific gravity 2.45 2.46 2.47 Young's modulus/specificgravity 44.1 39.8 38.5 (GPa) Bending strength (MPa) 430 700 500

[0259] TABLE 2 Examples 1-4 1-5 1-6 SiO₂ 75.0 75.0 76.0 Li₂O 9.9 9.5 9.5P₂O₅ 2.0 2.5 2.3 ZrO₂ 3.0 3.9 4.5 Al₂O₃ 7.0 7.0 6.0 MgO 0.4 — — ZnO 0.50.5 — K₂O 2.0 1.4 1.5 Sb₂O₃ 0.2 0.2 0.2 Nucleation temperature 520 540540 (° C.) Crystallization 730 740 780 temperature (° C.) Predominantcrystal phase lithium lithium lithium disilicate disilicate disilicateLi₂Si₂O₅ Li₂Si₂O₅ Li₂Si₂O₅ Average grain diameter 0.010 μm 0.020 μm0.040 μm (μm) Amount of crystal (mass %) 5 mass % 7 mass % 18 mass %Predominant crystal phase α-quartz α-cristobalite α-quartz α-SiO₂ α-SiO₂α-SiO₂ Average grain diameter 0.010 μm 0.010 μm 0.100 μm (μm) Amount ofcrystal (mass %) 23 mass % Predominant crystal phase α-cristobaliteα-SiO₂ Average grain diameter 0.050 μm (μm) Degree of crystallization 1014 50 (mass %) Coefficient of thermal 72 74 79 expansion (× 10⁻⁷/C)(−50° C. − +70° C.) Young's modulus (GPa) 105 100 97 Specific gravity2.46 2.47 2.50 Young's modulus/specific 42.7 40.5 38.8 gravity (GPa)Bending strength (MPa) 460 760 580

[0260] TABLE 3 Examples 1-7 1-8 1-9 SiO₂ 72.0 74.2 75.6 Li₂O 11.0 9.08.5 P₂O₅ 2.5 1.8 1.7 ZrO₂ 6.9 3.0 2.1 Al₂O₃ 3.6 7.8 8.5 MgO 0.5 1.7 ZnO1.0 K₂O 2.5 2.0 1.5 Sb₂O₃ 1.0 0.5 Nucleation temperature (° C.) 580 560590 Crystallization temperature 740 780 780 (° C.) Predominant crystalphase lithium lithium lithium disilicate disilicate disilicate Li₂Si₂O₅Li₂Si₂O₅ Li₂Si₂O₅ Average grain diameter (μm) 0.020 μm 0.030 μm 0.050 μmAmount of crystal (mass %) 20 mass % 12 mass % 18 mass % Predominantcrystal phase α-quartz α-quartz α-quartz α-SiO₂ α-SiO₂ α-SiO₂ Averagegrain diameter (μm) 0.030 μm 0.040 μm 0.050 μm Amount of crystal (mass%) 20 mass % Predominant crystal phase α-cristobalite α-SiO₂ Averagegrain diameter (μm) 0.040 μm Degree of crystallization 30 18 48 (mass %)Coefficient of thermal 80 100 130 expansion (× 10⁻⁷/° C.) (−50° C. −+70° C.) Young's modulus (GPa) 110 100 96 Specific gravity 2.44 2.422.48 Young's modulus/specific 45.1 41.3 38.7 gravity (GPa) Bendingstrength (MPa) 650 600 420

[0261] TABLE 4 Examples 1-10 1-11 1-12 SiO₂ 74.3 77.2 78.4 MgO 0.8 1.00.5 Al₂O₃ 7.0 3.5 2.5 P₂O₅ 2.0 2.0 2.0 Li₂O 9.9 10.6 10.7 ZnO 0.5 0.50.5 ZrO₂ 2.3 2.4 2.2 TiO₂ Sb₂O₃ 0.2 0.2 0.2 As₂O₃ K₂O 3.0 2.6 3.0 MoO₃Nucleation temperature 540 520 540 (° C.) Crystallization temperature770 750 780 (° C.) Predominant crystal phase α-quartz lithium lithiumα-SiO₂ disilicate disilicate Li₂Si₂O₅ Li₂Si₂O₅ Average grain diameter0.15 μm 0.10 μm 0.10 μm (μm) Amount of crystal (mass %) 25 mass % 55mass % 43 mass % Predominant crystal phase lithium α-cristobaliteα-cristobalite disilicate α-SiO₂ α-SiO₂ Li₂Si₂O₅ Average grain diameter0.10 μm 0.15 μm 0.15 μm (μm) Degree of crystallization 50 65 60 (mass %)Coefficient of thermal 115 87 75 expansion (× 10⁻⁷/° C.) (−50° C.-+70°C.) Young's modulus (GPa) 98 100 105 Specific gravity 2.42 2.42 2.41Young's modulus/specific 40.5 41.3 43.6 gravity (GPa)

[0262] TABLE 5 Examples 1-13 1-14 SiO₂ 76.5 76.0 MgO 0.8 0.9 Al₂O₃ 3.54.5 P₂O₅ 2.3 2.5 K₂O 3.8 2.5 Li₂O 10.5 10.0 ZnO 0.5 0.4 ZrO₂ 1.9 3.0Sb₂O₃ 0.2 0.2 As₂O₃ Nucleation temperature (° C.) 540 560Crystallization temperature (° C.) 770 750 Predominant crystal phaseα-quartz lithium disilicate α-SiO₂ Li₂Si₂O₅ Average grain diameter (μm)0.15 μm 0.040 μm Amount of crystal (mass %) 20 mass % 8 mass %Predominant crystal phase lithium α-quartz disilicate α-SiO₂ Li₂Si₂O₅Average grain diameter (μm) 0.10 μm 0.040 μm Degree of crystallization(mass %) 50 24 Coefficient of thermal expansion 110 75 (× 10⁻⁷/° C.)(−50° C. − +70° C.) Young's modulus (GPa) 100 108 Specific gravity 2.432.48 Young's modulus/specific gravity 41.2 43.5 (GPa)

[0263] TABLE 6 Examples 3-1 3-2 3-3 SiO₂ 53.5 53.5 53.5 MgO 15.0 15.015.0 Al₂O₃ 18.0 18.0 18.0 P₂O₅ 2.0 2.0 2.0 Li₂O CaO 2.0 2.0 2.0 BaO 2.02.0 2.0 TiO₂ 7.0 7.0 6.5 Sb₂O₃ As₂O₃ 0.5 0.5 0.2 Others Fe₂O₃ 0.8Nucleation temperature 700 700 650 (° C.) Crystallization 970 980 830temperature (° C.) Predominant crystal phase cordierite cordieriteenstatite Mg₂Al₄Si₅O₁₈ Mg₂Al₄Si₅O₁₈ MgSiO₃ Average grain diameter 0.3 μm0.3 μm 0.10 μm (μm) Amount of crystal 55 mass % 60 mass % 50 mass %(mass %) Predominant crystal phase β-quartz SS β-quartz SS β-SiO₂SSβ-SiO₂SS Average grain diameter 0.10 μm 0.10 μm (μm) Degree ofcrystallization 65 70 50 (mass %) Coefficient of thermal 37 35 46expansion (× 10⁻⁷/° C.) (−50° C. − +70° C.) Young's modulus (GPa) 135145 113 Specific gravity 2.72 2.80 2.58 Young's modulus/specific 49.651.8 43.8 gravity (GPa)

[0264] TABLE 7 Examples 3-4 3-5 3-6 SiO₂ 49.0 45.5 52.7 MgO 14.0 17.015.0 Al₂O₃ 17.0 19.0 16.0 CaO 1.7 1.2 1.4 SrO 1.7 1.2 1.4 BaO 4.2 1.43.5 ZrO₂ 1.4 0.8 TiO₂ 9.0 9.5 9.0 Bi₂O₃ 1.8 5.0 Sb₂O₃ 0.2 0.2 0.2Nucleation temperature 700 700 700 (° C.) Crystallization 970 950 950temperature (° C.) Predominant crystal phase enstatite enstatiteenstatite MgSiO₃ MgSiO₃ MgSiO₃ magnesium spinel SS magnesium titanate SSMgAl₂O₄ titanate SS MgTi₂O₅SS MgTi₂O₅SS Average grain diameter 0.1 μm0.1 μm 0.1 μm (μm) Coefficient of thermal 51 51 51 expansion (× 10⁻⁷/°C.) (−50° C. − +70° C.) Young's modulus (GPa) 116 135 120 Specificgravity 2.97 2.97 2.91 Young's modulus/specific 39.1 45.5 41.2 gravity(GPa)

[0265] TABLE 8 Examples 3-7 3-8 SiO₂ 53.5 53.5 MgO 15.0 15.0 Al₂O₃ 18.018.0 P₂O₅ 2.0 2.0 Li₂O CaO 2.0 2.0 ZrO₂ 1.0 TiO₂ 7.0 7.0 Sb₂O₃ As₂O₃ 0.50.5 BaO 1.0 1.0 MoO₃ 1.0 Nucleation temperature (° C.) 700 700Crystallization temperature (° C.) 970 980 Predominant crystal phasecordierite cordierite Mg₂Al₄Si₅O₁₈ Mg₂Al₄Si₅O₁₈ Average grain diameter(μm) 0.3 μm 0.3 μm 0.3 μm Amount of crystal (mass %) 63 mass % 68 mass %Predominant crystal phase β-quartz SS spinel β-SiO₂SS MgAl₂O₄ Averagegrain diameter (μm) 0.10 μm 0.10 μm Degree of crystallization (mass %)65 70 Coefficient of thermal expansion 37 45 (× 10⁻⁷/° C.)(−50° C. −+70° C.) Young's modulus (GPa) 135 155 Specific gravity 2.78 2.80Young's modulus/specific gravity 48.6 55.3 (GPa)

[0266] TABLE 9 Comparative Examples 1 2 3 SiO₂ 43.0 75.8 55.0 MgO 23.02.0 10.0 Al₂O₃ 26.8 2.5 10.0 P₂O₅ 2.5 ZnO 0.5 10.0 Li₂O K₂O = 2.4 10.0ZrO₂ 1.5 Sb₂O₃ 0.2 As₂O₃ = 0.3 Others Ga₂O₃ = 4.8 K₂O = 3.2 Nb₂O₅ = 3.0V₂O₅ = 0.6 CaO = 3.0 MnO₂ = 0.6 TiO₂ = 3.7 CuO = 0.6 BaO = 5.0Nucleation temperature 800 540 720 (° C.) Crystallization 950 700 880temperature (° C.) Predominant crystal phase spinel lithium gahniteMgAl₂O₄ disilicate ZnAl₂O₄ Li₂Si₂O₅ Average grain 0.10 μm 0.10 μm 0.05μm diameter (μm) Amount of crystal 75 mass % 30 mass % 40 mass % (mass%) Degree of crystallization 75 30 40 (mass %) Coefficient of thermal 5360 55 expansion (× 10⁻⁷/° C.) (−50° C. − +70° C.) Young's modulus (GPa)110.5 90 120 Specific gravity 3.24 2.50 3.05 Young's modulus/specific34.1 36.0 39.3 gravity (GPa)

[0267] The spacer rings for the information storage disk made of theglass-ceramics of Examples 1-1 to 1-14 and 3-1 to 3-8 and those ofComparative Examples 1-3 were manufactured by the following process.First, raw materials such as oxides, carbonates and nitrates wereweighed and mixed to compose the oxide compositions of the tables andthe mixtures were melted at a temperature within a range from about1350° C. to 1500° C. by using a conventional melting device andhomogenized by stirring. The melt was formed to a ring-shape and cooledto provide a glass form. It was heat treated under a temperature of450-800° C. for about 1 to 7 hours for nucleation and further heattreated under a temperature of 700-980° C. for about 1 to 7 hours forcrystallization. Further, the upper and lower surfaces of the productwere lapped and polished to provide Ra of 0.1 μm and flatness of 0.3 μmwhereby spacer rings for an information storage disk made of desiredglass-ceramics in which a crystal phase was uniformly dispersed in glassmatrix were obtained

[0268] As shown in Tables 1 to 9, the spacer rings of ComparativeExamples which are made of non-crystalline glass etc. have specificrigidity of less than 37 GPa or specific gravity exceeding 3.0 andtherefore are not suitable for a high speed rotation type disk drivedevice and, in contrast thereto, the spacer rings made of theglass-ceramics of the invention have a relatively small specific gravityand a relatively high specific rigidity and thereby have capability ofcoping with the tendency to high speed rotation of the disk drivedevice. In Examples 1-1 to 1-14 which are examples of spacer rings madeof glass-ceramics which contain lithium disilicate (Li₂O.2SiO₂) as apredominant crystal phase, the coefficient of thermal expansion within arange from −50° C. to +70° C. which is a temperature environment inwhich the product is used is within a range from +65×10⁻⁷/° C. to+130×10⁻⁷/° C. Thus, these Examples have a coefficient of thermalexpansion which is substantially equal to typical coefficient of thermalexpansion of other drive components for an information storage disk.

[0269] The bending strength of Examples 1-1 to 1-9 was calculated by thecup type ring bending test on the basis of inner diameter, outerdiameter, plate thickness, Poisson's ratio and maximum load with respectto a spacer ring of a ring shape having inner diameter of about 20.0 mm,outer diameter of about 23.5 mm and plate thickness of about 2.0 mm. Thebending strength of the spacer rings of Examples 1-1 to 1-9 is within arange of 400 MPa-800 MPa and therefore is suitable for uses such asmobiles which require shock resistance property.

[0270] Preferred examples of the second aspect of the invention will bedescribed below. Tables 10 to 12 show compositions, nucleationtemperatures, crystallization temperatures, crystal phases, amount ofcrystal of the crystal phases and average crystal grain diameters ofExamples (2-1 to 2-7) of the information storage disk holding members ofthe invention and two prior art glass-ceramics (Comparative Example 8:Japanese Patent Application Laid-open Publication No. Sho 62-72547 andComparative Example 9: Japanese Patent Application Laid-open PublicationNo. Hei 9-35234). Tables 13 to 15 show coefficients of thermalexpansion, values of Young's modulus, specific gravity, bending strengthand alkali dissolving amount of the information storage disk holdingmembers of Tables 10 to 12. TABLE 10 Examples 2-1 2-2 2-3 SiO₂ 73.7 73.773.7 Li₂O 6.6 6.6 6.6 P₂O₅ 1.9 1.9 1.9 ZrO₂ 3.0 3.0 3.0 Al₂O₃ 8.0 8.08.0 MgO 0.5 0.5 0.5 ZnO 0.5 0.5 0.5 SrO 0.5 0.5 BaO 0.5 Y₂O₃ WO₃ 2.0La₂O₃ 2.0 Bi₂O₃ 2.0 K₂O 3.0 3.0 3.0 Na₂O Sb₂O₃ As₂O₃ 0.3 0.3 0.3Nucleation temperature (° C.) 540 540 540 Crystallization temperature (°C.) 740 750 760 Predominant crystal phase α-quartz lithium lithiumα-SiO₂ disilicate disilicate Li₂Si₂O₅ Li₂Si₂O₅ Average grain diameter(μm) 0.01 μm <0.01 μm 0.01 μm Amount of crystal (mass %) 10% 20% 25%Predominant crystal phase lithium α-quartz α-quartz disilicate α-SiO₂α-SiO₂ Li₂Si₂O₅ Average grain diameter (μm) <0.01 μm 0.01 μm 0.02 μmAmount of crystal (mass %) 15% 16% 20%

[0271] TABLE 11 Examples 2-4 2-5 2-6 SiO₂ 73.7 73.7 73.7 Li₂O 6.6 6.66.6 P₂O₅ 1.9 1.9 1.9 ZrO₂ 3.0 3.0 3.0 Al₂O₃ 8.0 8.0 8.0 MgO 0.5 0.5 0.5ZnO 0.5 0.5 0.5 SrO 0.5 0.5 0.5 BaO Y₂O₃ 2.0 WO₃ La₂O₃ 1.0 2.0 Bi₂O₃ 1.0K₂O 3.0 3.0 2.0 Na₂O 1.0 Sb₂O₃ As₂O₃ 0.3 0.3 0.3 Nucleation temperature(° C.) 440 440 500 Crystallization temperature (° C.) 720 740 760Predominant crystal phase α-quartz lithium lithium α-SiO₂ disilicatedisilicate Li₂Si₂O₅ Li₂Si₂O₅ Average grain diameter (μm) 0.01 μm <0.01μm 0.01 μm Amount of crystal (mass %) 10% 20% 35% Predominant crystalphase lithium α-quartz α-quartz disilicate α-SiO₂ α-SiO₂ Li₂Si₂O₅Average grain diameter (μm) <0.01 μm 0.01 μm 0.02 μm Amount of crystal(mass %) 15% 30% 33%

[0272] TABLE 12 Example Comparative Examples 2-7 4 5 SiO₂ 73.7 74.2 76.1Li₂O 6.6 9.6 11.8 P₂O₅ 2.0 1.5 2.0 ZrO₂ 3.0 0.4 Al₂O₃ 8.0 9.6 7.1 MgO0.5 PbO = 2.3 ZnO 0.5 SrO 0.5 BaO Y₂O₃ 0.5 WO₃ 0.5 La₂O₃ 0.5 Bi₂O₃ 0.5K₂O 2.0 2.4 2.8 Na₂O 0.9 Sb₂O₃ 0.2 As₂O₃ 0.3 Nucleation temperature 560540 500 (° C.) Crystallization temperature 750 800 860 (° C.)Predominant crystal phase lithium lithium lithium disilicate disilicatedisilicate Li₂Si₂O₅ La₂Si₂O₅ La₂Si₂O₅ Average grain diameter 0.01 μm 1.5μm 0.1 μm (μm) Amount of crystal (mass %) 28% 45% 48% Predominantcrystal phase α-quartz α- β- α-SiO₂ cristobalite spodumene Li₂Si₂O₅α-SiO₂ β-Li₂O Al₂O₃.4SiO₂ Average grain diameter 0.02 μm 0.3 μm 0.2 μm(μm) Amount of crystal (mass %) 25% 16% 21%

[0273] TABLE 13 Examples 2-1 2-2 2-3 Coefficient of thermal expansion 7295 110 (× 10⁻⁷/° C.)(-50° C. − +70° C.) Young's modulus (GPa) 110 114110 Specific gravity 2.55 2.56 2.57 Young's modulus(GPa)/specific 43.144.5 42.8 gravity Bending strength (MPa) 510 600 750 Alkali dissolvingamount Li (μg/Disk) 0.300 0.300 0.300 Na (μg/Disk) 0.005 0.005 0.005 K(μg/Disk) 0.090 0.080 0.090 Total (μg/Disk) 0.395 0.385 0.395 Total(μg/cm²) 0.0064 0.0062 0.0064

[0274] TABLE 14 Examples 2-4 2-5 2-6 Coefficient of thermal expansion 78115 125 (× 10⁻⁷/° C.)(−50° C.-+70° C.) Young's modulus (GPa) 115 120 110Specific gravity 2.52 2.55 2.58 Young's modulus(GPa)/specific 45.6 47.142.6 gravity Bending strength (MPa) 480 630 710 Alkali dissolving amountLi (μg/Disk) 0.400 0.300 0.200 Na (μg/Disk) 0.004 0.006 0.005 K(μg/Disk) 0.080 0.070 0.030 Total (μg/Disk) 0.484 0.376 0.235 Total(μg/cm²) 0.0078 0.0061 0.0038

[0275] TABLE 15 Comparative Example Example 2-7 4 5 Coefficient ofthermal expansion 119 48 49 (× 10⁻⁷/° C.)(−50° C. − +70° C.) Young'smodulus (GPa) 118 86 82 Specific gravity 2.57 2.46 2.55 Young'smodulus(GPa)/specific 45.9 35.0 32.2 gravity Bending strength (MPa) 720320 300 Alkali dissolving amount Li (μg/Disk) 0.100 2.800 3.000 Na(μg/Disk) 0.003 0.010 0.013 K (μg/Disk) 0.060 0.100 0.110 Total(μg/Disk) 0.163 2.910 3.123 Total (μg/cm²) 0.0026 0.0471 0.0506

[0276] Raw materials such as oxides, carbonates and nitrates were mixedand were melted at a temperature within a range from about 1350° C. to1450° C. by using a conventional melting device and homogenized bystirring. The melt was formed to a disk-shape and cooled to provide aglass form. It was heat treated under a temperature of 400-600° C. forabout 1 to 7 hours for nucleation and further heat treated under atemperature of 700-760° C. for about 1 to 7 hours for crystallization toprovide desired glass-ceramics.

[0277] Average crystal grain diameter of each crystal phase was measuredby a transmission electron microscope (TEND. Type of each crystal phasewas determined by X-ray diffraction analysis (XRD) device and amount ofcrystal was measured by preparing a 100% crystal standard specimen foreach crystal type and measuring a diffraction peak area by using theinternal standard method.

[0278] The bending strength was calculated by the cup type ring bendingtest on the basis of inner diameter, outer diameter, plate thickness,Poission's ratio and maximum load.

[0279] Measurement of alkali dissolving amount was conducted by usingion chromatography. Each information storage disk holding memberobtained in the above described manner was soaked in 80 ml of pure waterat 30° C. for 3 hours and the alkali dissolving amount of the substratewas calculated from concentration of alkali which solved out in the purewater.

[0280] As shown in Tables 10 to 12, the information storage disk holdingmembers of the present invention comprise lithium disilicate (Li₂Si₂O₅)and α-quartz as predominant crystal phases and have fine crystal grainsof 0.02 μm or below which are substantially spherical. In contrast, theglass-ceramic of Comparative Example 4 comprises lithium disilicatehaving an average crystal grain diameter of 1.5 μm and the glass-ceramicof Comparative Example 5 comprises β-spodumene having an average crystalgrain diameter of 0.2 μm and both have a relatively large acicular shapeor rice grain shape.

[0281] As to the coefficient of thermal expansion (×10⁻⁷/° C.),Comparative Examples 4 and 5 are low expansion glass-ceramics having acoefficient of less than 50. Further, the glass-ceramics of ComparativeExamples 4 and 5 are materials of low Young's modulus and low strength,showing Young's modulus of not greater than 84 GPa and bending strengthof not greater than 320 MPa.

[0282] Preferred examples of the fourth aspect of the invention will nowbe described. Tables 16 to 18 show compositions of Examples (4-1 to 4-9)of the information storage disk holding members of the invention andthose of Comparative Examples (Comparative Example 6: Japanese PatentApplication Laid-open Publication No. Hei 8-48537, the prior artalumino-silicate glass which is a type of chemically tempered glass,Comparative Example 7: Japanese Patent Application Laid-open PublicationNo. Hei 9-35234, Li₂O—SiO₂ glass-ceramics and Comparative Example 8:Japanese Patent Application Laid-open Publication No. Hei 9-77531,SiO₂═—Al₂O₃—MgO—ZnO—TiO₂ glass-ceramics) together with nucleationtemperature, crystallization temperature, crystal phase, crystal graindiameter, Young's modulus, Vickers' hardness, specific gravity andcoefficient of thermal expansion within a range of −50° C.-+70° C. Theprecipitation ratio of the respective crystal phases was measured bypreparing a 100% crystal standard specimen of each crystal phase andcalculating a diffraction peak area by an X-ray diffraction (XRD) deviceusing the internal standard method.

[0283] In Tables 16 to 18, magnesium titanate solid solution isexpressed as magnesium titanate SS, spinel solid solution as spinel SSand solid solutions of other crystals are expressed by affixing “SS” tothe names of the crystals (e.g., β-quartz solid solution is expressed asβ-quartz SS). TABLE 16 Examples 4-1 4-2 4-3 SiO₂ 49.0 49.0 51.0 MgO 14.015.0 14.0 Al₂O₃ 17.0 16.0 17.0 CaO 1.7 1.7 1.4 SrO 1.7 1.7 1.4 BaO 4.24.2 3.5 ZrO₂ 1.4 1.4 0.8 TiO₂ 9.0 9.0 9.0 Bi₂O₃ 1.8 1.8 1.7 Sb₂O₃ 0.20.2 0.2 Nucleation temperature:time 700° C.:5 Hr 700° C.:3 Hr 700° C.:8Hr Crystallization 900° C.:3 Hr 950° C.:7 Hr 950° C.:5 Hrtemperature:time Predominant crystal phase enstatite magnesium magnesiumMgSiO₃ titanate SS titanate SS magnesium MgTi₂O₅SS MgTi₂O₅SS titanate SSenstatite enstatite MgTi₂O₅SS MgSiO₃ MgSiO₃ Average grain diameter(μm)0.1 μm 0.1 μm 0.1 μm Young's modulus (GPa) 116 125 120 Vickers' hardness720 780 800 Specific gravity 2.97 3.03 2.95 Coefficient of thermal 51 4950 expansion (× 10⁻⁷/° C.) (−50° C. − +70° C.)

[0284] TABLE 17 Examples 4-4 4-5 4-6 SiO₂ 45.5 45.5 52.7 MgO 17.0 17.015.0 Al₂O₃ 19.0 19.0 16.0 CaO 1.2 1.2 1.4 SrO 1.2 1.2 1.4 BaO 1.4 1.43.5 ZrO₂ 0.8 TiO₂ 9.5 9.5 9.0 Bi₂O₃ 5.0 2.5 Sb₂O₃ 0.2 0.2 0.2 Otheringredient 2.5(WO₃) Nucleation temperature:time 700° C.:9 Hr 700° C.:3Hr 700° C.:5 Hr Crystallization 900° C.:5 Hr 950° C.:5 Hr 950° C.:2 Hrtemperature:time Predominant crystal phase enstatite enstatite enstatiteMgSiO₃ MgSiO₃ MgSiO₃ spinel SS spinel SS magnesium MgAl₂O₄SS MgAl₂O₄SStitanate SS MgT₂0₅SS Average grain diameter (μm) 0.1 μm 0.1 μm 0.1 μmYoung's modulus (GPa) 135 123 120 Vickers' hardness 800 810 820 Specificgravity 2.97 3.10 2.91 Coefficient of thermal 51 48 51 expansion (×10⁻⁷/° C.) (−50° C. − +70° C.)

[0285] TABLE 18 Examples 4-7 4-8 4-9 SiO₂ 55.0 51.0 44.0 MgO 10.0 15.018.0 Al₂O₃ 17.5 16.0 19.0 CaO 1.2 1.4 1.0 SrO 1.2 1.4 1.0 BaO 1.9 3.52.5 ZrO₂ 0.8 TiO₂ 9.5 9.0 10.0 Bi₂O₃ 3.0 1.7 4.0 Sb₂O₃ 0.2 0.2 0.5 Otheringredient Nucleation 700° C.:3 Hr  700° C.:5 Hr 700° C.:5 Hrtemperature:time Crystallization 950° C.:5 Hr 1000° C.:1 Hr 900° C.:5 Hrtemperature:time Predominant crystal phase magnesium enstatite enstatitetitanate SS MgSiO₃ MgSiO₃ MgTi₂O₃SS magnesium spinel SS enstatitetitanate SS MgAl₂O₄SS MgSiO₃ MgTi₂O₅SS β-quartz Ss rutile β-SiO₂ SS TiO₂Average grain diameter 0.1-0.2 μm 0.1-0.3 μm 0.1 μm (μm) Young's modulus(GPa) 130 123 148 Vickers' hardness 810 840 790 Specific gravity 2.882.97 3.15 Coefficient of thermal 53 50 50 expansion (× 10⁻⁷/° C.) (−50°C. − +70° C.)

[0286] TABLE 19 Comparative Examples 6 7 8 SiO₂ 62.0 78.5 49.0 MgO 12.0Al₂O₃ 16.0 4.4 24.8 P₂O₅ 2.0 ZnO 5.0 TiO₂ 10.0 Li₂O 7.0 12.5 Otheralkali ingredients 9.0(Na₂O) 2.8(K₂O) ZrO₂ 4.0 Sb₂O₃ 0.5 0.2 Otheringredient As₂O₃ = 0.5 Nucleation temperature:time 450° C.:5 Hr 700°C.:5 Hr Crystallization 850° C.:5 Hr 965° C.:5 Hr temperature:timePredominant crystal phase lithium spinel dsilicate MgAl₂O₄ Li₂Si₂O₅Average grain diameter 0.10 μm 0.10 μm Amount of crystal 25 mass%Predominant crystal phase α-cristobalite enstatite α-SiO₂ MgSiO₃ Averagegrain diameter 0.30 μm 0.10 μm Degree of crystallization 0 45 (mass %)Young's modulus (GPa) 82 92 119 Specific gravity 2.54 2.51 2.87 Vickers'hardness 640 760 1000 Coefficient of thermal 70 61 53 expansion (×10⁻⁷/° C.) (−50° C. − +70° C)

[0287] Raw materials such as oxides, carbonates and nitrates were mixedand were melted at a temperature within a range from about 1350° C. to1490° C. by using a conventional melting device and homogenized bystirring. The melt was formed to a disk-shape and cooled to provide aglass form. It was heat treated under a temperature of 650-750° C. forabout 1 to 12 hours for nucleation and further heat treated under atemperature of 850-1000° C. for about 1 to 12 hours for crystallizationto provide desired glass-ceramics.

[0288] As shown in Tables 16 to 19, the glass-ceramics of the presentinvention are different in the crystal phases from Comparative Examplesof the prior art alumino-silicate chemically tempered glass, Li₂O—SiO₂glass-ceramics and SiO₂—Al₂O₃—MgO—ZnO—TiO₂ glass-ceramics and are ofhigher rigidity in terms of Young's modulus than the alumino-silicatechemically tempered glass and the Li₂O—SiO₂ glass-ceramics. The SiO₂—Al₂O₃—MgO—ZnO—TiO₂ glass-ceramics of Comparative Example 8 are very hardmaterial having Vickers' hardness of 1000 (9800N/mm²) as the surfacehardness and therefore are difficult to process. In contrast, theglass-ceramics of the invention have Vickers' hardness of 850(8330N/mm²) as the surface hardness and sufficient smoothness can beobtained in normal polishing. Moreover, the glass-ceramics of theinvention have no defects such as crystal anisotropy, foreign mattersand impurities, have a dense, homogeneous and fine (the grain diameterof the precipitating crystals is 0.3 μm or below) and have chemicaldurability by which the glass-ceramics can stand rinsing or etching withvarious chemicals and water.

[0289] Preferred examples of the fifth aspect of the invention will nowbe described. Tables 20 to 24 show examples (5-1 to 5-14) ofcompositions, nucleation temperature, crystallization temperature,crystal phase, crystal grain diameter, Young's modulus, specificgravity, Young's modulus (GPa)/specific gravity, and coefficient ofthermal expansion within a range of −50° C.-+70° C. of the informationstorage disk holding members of the invention. β-quartz solid solutionis expressed as β-quartz SS. TABLE 20 Examples 5-1 5-2 5-3 SiO₂ 52.252.2 52.2 MgO 12.0 12.0 12.0 Al₂O₃ 17.0 16.0 16.0 P₂O₅ 1.5 1.5 1.5 B₂O₃3.0 3.5 4.0 Li₂O 2.0 2.5 2.0 CaO 2.0 2.0 2.0 ZrO₂ 2.0 1.5 1.5 TiO₂ 5.05.5 5.5 Sb₂O₃ 0.3 0.3 0.3 As₂O₃ SnO₂ 1.5 1.5 1.5 MoO₃ 1.5 1.5 1.5 CeOFe₂O₃ Nucleation temperature 650 600 650 Crystallization temperature1000 800 900 Predominant crystal phase β-quartz SS β-quartz SS β-quartzSS Average grain diameter  0.1 μm  0.1 μm  0.1 μm Predominant crystalphase enstatite enstatite enstatite Average grain diameter 0.05 μm 0.05μm 0.05 μm Young's modulus (GPa) 120 128 135 Specific gravity 2.65 2.652.67 Young's modulus(GPa)/ 45.3 48.3 50.6 specific gravity Coefficientof thermal 43 48 45 expansion (× 10⁻⁷/° C.) (−50° C. − +70° C.)

[0290] TABLE 21 Examples 5-4 5-5 5-6 SiO₂ 52.2 56.0 41.0 MgO 12.0 13.016.0 Al₂O₃ 16.0 15.0 20.0 P₂O₅ 1.5 1.5 2.0 B₂O₃ 3.5 3.5 4.0 Li₂O 2.0 2.52.0 CaO 2.5 3.5 3.9 ZrO₂ 1.5 1.0 2.5 TiO₂ 5.5 3.5 3.5 Sb₂O₃ 0.3 0.5 0.2As₂O₃ SnO₂ 1.5 4.9 MoO₃ 1.5 CeO Fe₂O₃ Nucleation temperature 700 650 670Crystallization temperature 1000 750 800 Predominant crystal phaseβ-quartz SS β-quartz SS β-quartz SS Average grain diameter  0.1 μm  0.1μm  0.1 μm Predominant crystal phase enstatite enstatite enstatiteAverage grain diameter 0.05 μm 0.05 μm 0.05 μm Young's modulus (GPa) 157120 122 Specific gravity 2.71 2.50 2.53 Young's modulus(GPa)/ 57.9 48.048.2 specific gravity Coefficient of thermal 41 50 48 expansion (×10⁻⁷/° C.) (−50° C. − +70° C.)

[0291] TABLE 22 Examples 5-7 5-8 5-9 SiO₂ 51.4 46.1 49.0 MgO 11.0 18.019.0 Al₂O₃ 18.0 15.0 11.0 P₂O₅ 2.0 0.5 2.5 B₂O₃ 1.4 1.5 3.5 Li₂O 0.5 3.53.9 CaO 2.5 3.5 0.5 ZrO₂ 0.5 1.0 4.7 TiO₂ 7.7 7.5 2.5 Sb₂O₃ 0.01 0.010.1 As₂O₃ 0.09 0.39 0.3 SnO₂ 1.5 1.5 MoO₃ 1.5 1.5 CeO 4.9 Fe₂O₃Nucleation temperature 690 710 730 Crystallization temperature 850 9001000 Predominant crystal phase β-quartz SS β-quartz SS β-quartz SSAverage grain diameter  0.1 μm  0.1 μm  0.1 μm Predominant crystal phaseenstatite enstatite enstatite Average grain diameter 0.05 μm 0.05 μm0.20 μm Young's modulus (GPa) 128 130 140 Specific gravity 2.62 2.702.80 Young's modulus(GPa)/ 48.9 48.1 50.0 specific gravity Coefficientof thermal 47 45 37 expansion (× 10⁻⁷/° C.) (−50° C. − +70° C.)

[0292] TABLE 23 Examples 5-10 5-11 5-12 SiO₂ 59.7 56.8 60.0 MgO 13.014.0 10.5 Al₂O₃ 12.0 14.0 16.0 P₂O₅ 1.0 1.0 1.0 B₂O₅ 2.5 2.0 1.0 Li₂O1.0 1.0 0.7 CaO 0.5 1.0 1.0 ZrO₂ 3.0 0.5 3.5 TiO₂ 4.0 4.5 6.0 Sb₂O₃ 0.30.3 0.3 As₂O₃ SnO₂ MoO₃ 3.0 CeO Fe₂O₃ 4.9 Nucleation temperature 750 650680 Crystallization temperature 950 980 1050 Predominant crystal phaseβ-quartz SS β-Quartz SS β-quartz SS Average grain diameter  0.1 μm  0.1μm  0.1 μm Predominant crystal phase enstatite enstatite enstatiteAverage grain diameter 0.05 μm 0.05 μm 0.10 μm Young's modulus (GPa) 141135 148 Specific gravity 2.81 2.78 2.90 Young's modulus(GPa)/ 50.2 48.648.6 specific gravity Coefficient of thermal 41 39 38 expansion (×10⁻⁷/° C.) (−50° C. − +70° C.)

[0293] TABLE 24 Examples 5-13 5-14 SiO₂ 49.2 41.0 MgO 12.0 16.0 Al₂O₃17.0 20.0 P₂O₅ 1.5 2.0 B₂O₃ 3.0 4.0 Li₂O 2.0 2.0 CaO 2.0 3.9 ZrO₂ 2.02.5 TiO₂ 5.0 3.5 Sb₂O₃ 0.3 0.2 As₂O₃ SnO₂ 1.5 4.9 MoO₃ 1.5 CeO 3.0 Fe₂O₃Nucleation temperature 650 670 Crystallization temperature 900 850Predominant crystal phase β-quartz SS β-quartz SS Average grain diameter 0.1 μm  0.1 μm Predominant crystal phase enstatite forsterite Averagegrain diameter 0.05 μm 0.05 μm Young's modulus (GPa) 128 125 Specificgravity 2.75 2.58 Young's modulus(GPa)/ 46.5 48.4 specific gravityCoefficient of thermal expansion 40 48 (× 10⁻⁷/° C.) (−50° C. − +70° C.)

[0294] In the examples of Tables 20 to 24, raw materials such as oxides,carbonates and nitrates were mixed and were melted at a temperaturewithin a range from about 1350° C. to 1490° C. by using a conventionalmelting device and homogenized by stirring. The melt was formed to adisk-shape and cooled to provide a glass form. It was heat treated undera temperature of 650-750° C. for about 1 to 12 hours for nucleation andfurther heat treated under a temperature of 750-1050° C. for about 1 to12 hours for crystallization to provide desired glass-ceramics.

[0295] As shown in Tables 20 to 24, the glass-ceramics of the inventionhave excellent processability and desired smoothness can be sufficientlyachieved and, further, have no defects such as crystal anisotropy,foreign matters and impurities, have a dense, homogeneous and finetexture and have chemical durability by which the glass-ceramics canstand rinsing or etching with various chemicals and water.

[0296] Preferred examples of the sixth aspect of the invention will nowbe described. Tables 25 to 33 show examples (6-1 to 6-27) ofcompositions, nucleation temperature, crystallization temperature,crystal phase, crystal grain diameter, Young's modulus, specificgravity, Young's modulus (GPa)/specific gravity, and coefficient ofthermal expansion within a range of 50° C.-+70° C. of the informationstorage disk holding members of the invention. B-quartz solid solutionis expressed as β-quartz SS. TABLE 25 Examples 6-1 6-2 6-3 SiO₂ 54.056.8 57.5 MgO 14.0 16.0 16.0 Al₂O₃ 19.5 17.0 14.0 P₂O₅ 1.0 B₂O₃ 1.0 2.0CaO 2.0 2.2 2.5 BaO 2.0 ZrO₂ 0.5 TiO₂ 5.0 5.5 6.0 Sb₂O₃ As₂O₃ 0.5 0.50.5 F Fe₂O₃ 3.0 1.0 Nucleation temperature 670 700 680 Crystallizationtemperature 1000 950 930 Predominant crystal phase cordierite cordieritecordierite Average grain diameter 0.3 μm 0.3 μm 0.3 μm Predominantcrystal phase spinel spinel enstatite Average grain diameter 0.1 μm 0.1μm 0.1 μm Young's modulus (GPa) 135 128 143 Specific gravity 2.65 2.602.58 Young's modulus(GPa)/ 50.9 49.2 55.4 specific gravity Coefficientof thermal 37 35 46 expansion (× 10⁻⁷/° C.) (−50° C. − +70° C.)

[0297] TABLE 26 Examples 6-4 6-5 6-6 SiO₂ 57.1 45.6 56.0 MgO 14.0 18.010.4 Al₂O₃ 17.9 19.5 15.0 P₂O₅ 3.0 B₂O₃ 3.0 2.5 3.0 CaO 2.0 2.0 3.8 BaO0.5 ZrO₂ TiO₂ 5.5 8.0 7.0 Sb₂O₃ As₂O₃ 0.5 0.9 F Fe₂O₃ 4.8 Nucleationtemperature 650 650 720 Crystallization temperature 980 940 1000Predominant crystal phase cordierite cordierite cordierite Average graindiameter 0.3 μm 0.3 μm 0.3 μm Predominant crystal phase β-quartzβ-quartz β-quartz Average grain diameter 0.1 μm 0.1 μm 0.1 μm Young'smodulus (GPa) 133 121 141 Specific gravity 2.64 2.50 2.90 Young'smodulus(GPa)/ 50.4 48.4 48.6 specific gravity Coefficient of thermal 3048 36 expansion (× 10⁻⁷/° C.) (−50° C. − +70° C.)

[0298] TABLE 27 Examples 6-7 6-8 6-9 SiO₂ 59.5 60.0 41.7 MgO 17.0 19.517.0 Al₂O₃ 12.0 10.9 18.7 P₂O₅ 3.0 0.9 3.9 B₂O₃ 2.0 1.5 1.0 CaO 0.5 3.8BaO 2.0 ZrO₂ 2.8 TiO₂ 4.0 3.5 6.6 Sb₂O₃ 0.9 0.5 As₂O₃ F 2.8 Fe₂O₃ 4.0Nucleation temperature 700 730 750 Crystallization temperature 960 10001050 Predominant crystal phase cordierite cordierite cordierite Averagegrain diameter 0.3 μm 0.3 μm 0.3 μm Predominant crystal phase β-quartzβ-quartz β-quartz Average grain diameter 0.1 μm 0.1 μm 0.1 μm Young'smodulus (GPa) 138 120 151 Specific gravity 2.81 2.60 2.65 Young'smodulus(GPa)/ 49.1 46.2 57.0 specific gravity Coefficient of thermal 3241 38 expanision (× 10⁻⁷/° C.) (−50° C. − +70° C.)

[0299] TABLE 28 Examples 6-10 6-11 6-12 SiO₂ 55.5 50.0 56.2 MgO 18.013.0 11.0 Al₂O₃ 10.0 18.0 14.5 P₂O₅ 1.0 3.1 2.0 B₂O₃ 3.5 3.9 3.0 CaO 3.01.0 4.0 BaO 1.0 3.0 4.8 ZrO₂ 5.0 TiO₂ 2.5 7.5 4.0 Sb₂O₃ 0.5 0.5 0.5As₂O₃ F Fe₂O₃ Nucleation temperature 750 670 670 Crystallizationtemperature 1050 900 1050 Predominant crystal phase cordieritecordierite cordierite Average grain diameter 0.3 μm 0.1 μm 0.3 μmPredominant crystal phase β-quartz β-quartz β-quartz Average graindiameter 0.1 μm 0.1 μm 0.1 μm Young's modulus (GPa) 143 120 122 Specificgravity 2.90 3.10 2.70 Young's modulus(GPa)/ 49.3 38.7 45.2 specificgravity Coefficient of thermal 36 42 41 expansion (× 10⁻⁷/° C.) (−50° C.− +70° C.)

[0300] TABLE 29 Examples 6-13 6-14 6-15 SiO₂ 55.0 53.5 49.2 MgO 15.015.0 12.0 Al₂O₃ 17.0 18.0 17.0 P₂O₅ 1.0 2.0 1.5 B₂O₃ 2.0 3.0 Li₂O 2.0CaO 1.0 2.0 2.0 BaO 4.0 2.0 ZrO₂ 2.0 TiO₂ 4.5 7.0 5.0 Sb₂O₃ 0.5 0.3As₂O₃ 0.5 SnO₂ 1.5 MoO₃ 1.5 CeO 3.0 Fe₂O₃ Nucleation temperature 700 700650 Crystallization temperature 950 970 900 Predominant crystal phasecordierite cordierite β-quartz SS Average grain diameter 0.3 μm 0.3 μm 0.1 μm Predominant crystal phase β-quartz β-quartz enstatite Averagegrain diameter 0.1 μm 0.1 μm 0.05 μm Young's modulus (GPa) 128 134 128Specific gravity 2.80 2.77 2.75 Young's modulus(GPa)/ 44.4 48.4 46.5specific gravity Coefficient of thermal 37 30 40 expansion (× 10⁻⁷/° C.)(−50° C. − +70° C.)

[0301] TABLE 30 Examples 6-16 6-17 6-18 SiO₂ 52.2 52.2 52.2 MgO 12.012.0 12.0 Al₂O₃ 17.0 16.0 16.0 P₂O₅ 1.5 1.5 1.5 B₂O₃ 3.0 3.5 4.0 Li₂O2.0 2.5 2.0 CaO 2.0 2.0 2.0 ZrO₂ 2.0 1.5 1.5 TiO₂ 5.0 5.5 5.5 Sb₂O₃ 0.30.3 0.3 As₂O₃ SnO₂ 1.5 1.5 1.5 MoO₃ 1.5 1.5 1.5 CeO 3.0 Fe₂O₃ Nucleationtemperature 650 600 650 Crystallization 1000 800 900 temperaturePredominant crystal phase β-quartz SS β-quartz SS β-quartz SS Averagegrain diameter  0.1 μm  0.1 μm  0.1 μm Predominant crystal phaseenstatite enstatite enstatite Average grain diameter 0.05 μm 0.05 μm0.05 μm Young's modulus (GPa) 120 128 135 Specific gravity 2.65 2.652.67 Young's modulus(GPa)/ 45.3 48.3 50.6 specific gravity Coefficientof thermal 43 48 45 expansion (× 10⁻⁷/° C.) (−50° C. − +70° C.)

[0302] TABLE 31 Examples 6-19 6-20 6-21 SiO₂ 52.2 56.0 41.0 MgO 12.013.0 16.0 Al₂O₃ 16.0 15.0 20.0 P₂O₅ 1.5 1.5 2.0 B₂O₃ 3.5 3.5 4.0 Li₂O2.0 2.5 2.0 CaO 2.5 3.5 3.9 ZrO₂ 1.5 1.0 2.5 TiO₂ 5.5 3.5 3.5 Sb₂O₃ 0.30.5 0.2 As₂O₃ SnO₂ 1.5 4.9 MoO₃ 1.5 CeO Fe₂O₃ Nucleation temperature 700650 670 Crystallization 1000 750 800 temperature Predominant crystalphase β-quartz SS β-quartz SS β-quartz SS Average grain diameter  0.1 μm 0.1 μm  0.1 μm Predominant crystal phase enstatite enstatite enstatiteAverage grain diameter 0.05 μm 0.05 μm 0.05 μm Young's modulus (GPa) 157120 122 Specific gravity 2.71 2.50 2.53 Young's modulus(GPa)/ 57.9 48.048.2 specific gravity Coefficient of thermal 41 50 48 expansion (×10⁻⁷/° C.) (−50° C. − +70° C.)

[0303] TABLE 32 Examples 6-22 6-23 6-24 SiO₂ 51.4 46.1 49.0 MgO 11.018.0 19.0 Al₂O₃ 18.0 15.0 11.0 P₂O₅ 2.0 0.5 2.5 B₂O₃ 1.4 1.5 3.5 Li₂O2.5 3.5 3.9 CaO 2.5 3.5 0.5 ZrO₂ 0.5 1.0 4.7 TiO₂ 7.7 7.5 2.5 Sb₂O₃ 0.010.01 0.1 As₂O₃ 0.09 0.39 0.3 SnO₂ 1.5 1.5 MoO₃ 1.5 1.5 CeO 4.9 Fe₂O₃Nucleation temperature 690 710 730 Crystallization 850 900 1000temperature Predominant crystal phase β-quartz SS β-quartz SS β-quartzSS Average grain diameter  0.1 μm  0.1 μm  0.1 μm Predominant crystalphase enstatite enstatite enstatite Average grain diameter 0.05 μm 0.05μm 0.20 μm Young's modulus (GPa) 128 130 140 Specific gravity 2.62 2.702.80 Young's modulus(GPa)/ 48.9 48.1 50.0 specific gravity Coefficientof thermal 47 45 37 expansion (× 10⁻⁷/° C.) (−50° C. − +70° C.)

[0304] TABLE 33 Examples 6-25 6-26 6-27 SiO₂ 59.7 56.8 60.0 MgO 13.014.0 10.5 Al₂O₃ 12.0 14.0 16.0 P₂O₅ 1.0 1.0 1.0 B₂O₃ 2.5 2.0 1.0 Li₂O1.0 1.0 0.7 CaO 0.5 1.0 1.0 ZrO₂ 3.0 0.5 3.5 TiO₂ 4.0 4.5 6.0 Sb₂O₃ 0.30.3 0.3 As₂O₃ SnO₂ MoO₃ 3.0 CeO Fe₂O₃ 4.9 Nucleation temperature 750 650680 Crystallization 950 980 1050 temperature Predominant crystal phaseβ-quartz SS β-quartz SS β-quartz SS Average grain diameter  0.1 μm  0.1μm  0.1 μm Predominant crystal phase enstatite enstatite enstatiteAverage grain diameter 0.05 μm 0.05 μm 0.10 μm Young's modulus (GPa) 141135 148 Specific gravity 2.81 2.78 2.90 Young's modulus(GPa)/ 50.2 48.648.6 specific gravity Coefficient of thermal 41 39 38 expansion (×10⁻⁷/° C.) (−50° C. − +70° C.)

[0305] In the above examples, raw materials such as oxides, carbonatesand nitrates were mixed and were melted at a temperature within a rangefrom about 1350° C. to 1490° C. by using a conventional melting deviceand homogenized by stirring. The melt was formed to a disk-shape andcooled to provide a glass form. It was heat treated under a temperatureof 650-750° C. for about 1 to 12 hours for nucleation and further heattreated under a temperature of 750-1050° C. for about 1 to 12 hours forcrystallization to provide desired glass-ceramics.

[0306] As shown in Tables 25 to 33, the glass-ceramics of the inventionhave excellent processability and, further, have no defects such ascrystal anisotropy, foreign matters and impurities, have a dense,homogeneous and fine texture and have chemical durability by which theglass-ceramics can stand rinsing or etching with various chemicals andwater.

[0307] Preferred examples of the seventh and eighth aspects of theinvention will now be described. Tables 34 to 44 show examples (7-1 to7-27, 8-1 to 8-23) of compositions, nucleation temperature,crystallization temperature, crystal phase, crystal grain diameter andcoefficient of thermal expansion within a range of −60° C.- +600° C. ofthe information storage disk holding members of the invention. Table 45shows compositions and the above mentioned properties of the prior artAl₂O₃—SiO₂ glass-ceramics (Comparative Example No. 9) and the prior artLi₂O—SiO₂ glass-ceramics (Comparative Example No. 10). As to theprecipitated crystals in the tables, β-quartz is expressed as β-Q,β-quartz solid solution as β-Q-SS, β-spodumene as β-Sp, β-spodumenesolid solution as β-Sp-SS, β-eurcyptite as β-Eu, β-eucryptite solidsolution as β-Eu-SS, gahnite as Ga and gahnite solid solution as Ga—SS.TABLE 34 Examples 7-1 7-2 7-3 7-4 7-5 SiO₂ 55.0 54.0 54.0 30.5 64.8Al₂O₃ 18.5 18.0 18.0 20.0 12.0 ZnO 12.0 12.5 12.0 9.0 7.5 MgO 6.0 5.07.0 15.0 11.3 TiO₂ 6.0 4.5 5.0 3.0 1.7 B₂O₃ 2.5 7.5 ZrO₂ 1.0 P₂O₅ 1.0SnO₂ CaO 1.5 12.0 SrO 1.7 BaO 2.0 1.0 La₂O₃ Y₂O₃ 1.0 Gd₂O₃ Ta₂O₅ 1.0Nb₂O₅ WO₃ Bi₂O₃ V₂O₅ 1.0 As₂O₃ 0.5 0.5 0.5 1.0 0.5 Sb₂O₃ 1.0 0.5Nucleation temperature(° C.) 690 700 720 650 740 Crystallizationtemperature(° C.) 850 900 850 760 940 Predominant crystal phase Ga Ga GaGa-SS Ga-SS Average grain diameter(μm) 0.005 0.007 0.010 0.005 0.007Coefficient of thermal expansion (× 10⁻⁷/° C.) 49 55 65 70 33 (−60°C.-+600° C.)

[0308] TABLE 35 Examples 7-6 7-7 7-8 7-9 7-10 SiO₂ 60.0 31.5 55.0 37.050.0 Al₂O₃ 5.2 34.8 5.6 8.5 28.2 ZnO 10.0 20.0 5.3 34.5 10.0 MgO 2.5 4.52.0 5.5 1.1 TiO₂ 13.5 2.0 11.4 4.5 1.8 B₂O₃ 0.8 ZrO₂ 0.5 1.5 0.7 P₂O₅2.5 3.8 0.5 SnO₂ 0.9 CaO 5.0 SrO 5.5 BaO 6.0 13.0 La₂O₃ 2.5 Y₂O₃ Gd₂O₃1.0 6.9 Ta₂O₅ Nb₂O₅ 0.3 WO₃ Bi₂O₃ 0.7 V₂O₅ 1.0 As₂O₃ 0.5 Sb₂O₃ 0.5 0.52.0 Nucleation temperature(° C.) 670 650 650 650 750 Crystallizationtemperature(° C.) 850 750 830 900 800 Predominant crystal phase Ga- Ga-Ga- Ga-SS Ga-SS SS SS SS Average grain diameter(μm) 0.004 0.001 0.0010.010 0.001 Coefficient of thermal expansion (× 10⁻⁷/° C.) 55 80 60 7551 (−60° C.-+600° C.)

[0309] TABLE 36 Examples 7-11 7-12 7-13 7-14 7-15 SiO₂ 40.0 45.0 49.048.5 57.2 Al₂O₃ 25.0 15.0 17.0 22.5 25.5 ZnO 8.0 15.0 6.9 6.0 8.3 MgO19.5 18.0 3.0 3.1 1.5 TiO₂ 2.0 1.0 15.0 3.1 1.5 B₂O₃ 9.8 ZrO₂ 1.7 0.51.8 P₂O₅ 1.5 0.1 SnO₂ 0.1 CaO 2.5 SrO 0.5 BaO 1.3 0.7 La₂O₃ Y₂O₃ 3.0Gd₂O₃ 0.2 Ta₂O₅ Nb₂O₅ 4.0 WO₃ Bi₂O₃ 2.7 V₂O₅ As₂O₃ 0.5 2.0 3.5 Sb₂O₃ 0.53.0 3.5 Nucleation temperature(° C.) 700 700 650 650 700 Crystallizationtemperature(° C.) 850 850 750 750 890 Predominant crystal phase Ga- GaGa- Ga-SS Ga-SS SS SS Average grain diameter(μm) 0.001 0.001 0.001 0.0500.001 Predominant crystal phase Ga- SS Average grain diameter(μm) 0.001Coefficient of thermal expansion (× 10⁻⁷/° C.) 66 78 80 48 58 (−60°C.-+600° C.)

[0310] TABLE 37 Examples 7-16 7-17 7-18 7-19 7-20 SiO₂ 33.0 34.0 43.038.0 36.0 Al₂O₃ 9.5 31.0 10.0 30.0 8.5 ZnO 31.5 28.0 19.5 7.0 25.0 MgO9.5 1.3 1.9 1.7 8.0 TiO₂ 8.0 1.3 1.5 1.7 8.0 B₂O₃ ZrO₂ P₂O₅ 5.0 0.8 SnO₂2.0 CaO 0.4 19.8 SrO 20.0 BaO 19.5 La₂O₃ Y₂O₃ 1.5 Gd₂O₃ Ta₂O₅ 3.0 0.5Nb₂O₅ WO₃ Bi₂O₃ V₂O₅ AS₂O₃ 0.5 2.0 1.0 Sb₂O₃ 0.5 1.5 1.0 1.0 Nucleationtemperature(° C.) 650 650 650 650 660 Crystallization temperature(° C.)750 750 750 780 750 Predominant crystal phase Ga- Ga Ga- Ga-SS Ga-SSAverage grain diameter(μm) 0.001 0.001 0.100 0.050 0.001 Predominantcrystal phase Ga- SS Average grain diameter(μm) 0.001 Coefficient ofthermal expansion (× 10⁻⁷/° C.) 80 80 80 38 78 (−60° C.-+600° C.)

[0311] TABLE 38 Examples 7-21 7-22 7-23 7-24 7-25 SiO₂ 32.0 35.0 55.535.5 40.2 Al₂O₃ 28.0 10.0 12.2 30.5 11.2 ZnO 12.0 25.0 6.4 6.1 6.5 MgO4.0 6.2 2.5 1.6 16.0 TiO₂ 2.0 2.0 1.4 5.0 4.5 B₂O₃ 0.2 ZrO₂ P₂O₅ SnO₂CaO SrO BaO 3.0 La₂O₃ 19.5 Y₂O₃ 19.5 Gd₂O₃ 19.5 Ta₂O₅ 9.8 Nb₂O₅ 9.5 WO₃0.2 8.0 Bi₂O₃ 7.0 V₂O₅ As₂O₃ 2.5 3.5 2.1 Sb₂O₃ 1.8 2.5 Nucleationtemperature(° C.) 680 670 700 740 720 Crystallization temperature(° C.)770 760 800 870 880 Predominant crystal phase Ga- Ga- Ga- Ga-SS Ga-SS SSSS SS Average grain diameter(μm) 0.001 0.001 0.001 0.001 0.001Coefficient of thermal expansion (× 10⁻⁷/° C.) 76 37 61 48 51 (−60°C.-+600° C.)

[0312] TABLE 39 Examples 7-26 7-27 SiO₂ 50.8 52.0 Al₂O₃ 18.0 16.0 ZnO5.8 9.8 MgO 2.8 2.0 TiO₂ 2.2 2.0 B₂O₃ ZrO₂ P₂O₅ SnO₂ CaO SrO 2.0 BaOLa₂O₃ Y₂O₃ Gd₂O₃ Ta₂O₅ 7.8 Nb₂O₅ 7.4 WO₃ 9.5 Bi₂O₃ 9.5 V₂O₅ As₂O₃ 1.4Sb₂O₃ 1.5 Nucleation temperature (° C.) 710 900 Crystallizationtemperature (° C.) 950 900 Predominant crystal phase Ga-SS Ga-SS Averagegrain diameter (μm) 0.001 0.001 Coefficient of thermal expansion 80 54(× 10⁻⁷/° C.) (−60° C. − +600° C.)

[0313] TABLE 40 Examples 8-1 8-2 8-3 8-4 8-5 SiO₂ 55.0 53.5 56.5 50.561.5 P₂O₅ 8.0 8.0 7.5 9.0 6.0 Al₂O₃ 24.0 23.0 23.5 23.8 23.0 Li₂O 4.04.0 4.0 3.7 3.4 MgO 1.0 1.0 1.5 1.8 0.6 ZnO 0.5 0.5 0.5 1.0 1.0 CaO 1.01.0 0.3 2.5 0.5 BaO 1.0 1.0 0.7 2.5 0.5 TiO₂ 2.5 2.5 2.5 1.1 1.5 ZrO₂2.0 2.0 2.0 3.5 1.0 As₂O₃ 1.0 1.0 1.0 0.6 Sb₂O₃ 1.0 V₂O₅ 2.0 CoO 0.5Nucleation temperature(° C.) 750 730 700 650 730 Crystallizationtemperature(° C.) 800 900 850 770 900 Predominant crystal phase β -Q- β-Q- β -Q- β -Q-SS β -Q SS SS SS Average grain diameter(μm) 0.005 0.0320.050 0.005 0.010 Predominant crystal phase β -Sp β -Sp- SS Averagegrain diameter(μm) 0.100 0.010 Coefficient of thermal expansion (×10⁻⁷/° C.) 2 10 5 9 1 (−60° C.-+600° C.)

[0314] TABLE 41 Examples 8-6 8-7 8-8 8-9 8-10 SiO₂ 59.0 53.9 54.3 54.055.0 P₂O₅ 5.5 9.5 7.8 8.0 7.8 Al₂O₃ 24.0 22.8 22.5 25,8 23.0 Li₂O 4.73.0 3.4 3.4 3.2 MgO 0.8 0.7 1.7 1.4 0.7 ZnO 0.5 1.7 0.5 0.3 0.6 CaO 0.40.4 0.9 1.5 0.6 BaO 0.6 0.7 1.7 1.2 3.0 TiO₂ 1.3 3.2 1.9 2.2 1.4 ZrO₂1.3 1.5 1.5 1.3 2.2 AS₂O₃ 0.4 1.4 0.3 0.4 2.5 Sb₂O ₃ 1.5 1.2 3.5 0.5V₂O₅ CoO Nucleation temperature(° C.) 680 650 680 700 740Crystallization temperature(° C.) 950 760 780 820 850 Predominantcrystal phase β -Q- β -Q- β -Q β -Q-SS β -Q SS SS Average graindiameter(μm) 0.01 0.001 0.010 0.001 0.001 Predominant crystal phase β -Eu- SS Average grain diameter(μm) 0.010 Coefficient of thermal expansion(× 10⁻⁷/° C.) 4 −5 10 8 2 (−60° C.-+600° C.)

[0315] TABLE 42 Examples 8-16 8-17 8-18 8-19 8-50 SiO₂ 55.2 56.8 55.954.1 55.6 P₂O₅ 8.1 7.7 7.2 8.0 8.2 Al₂O₃ 22.7 25.4 24.8 24.1 24.5 Li₂O4.9 3.5 3.5 3.4 3.2 MgO 0.6 0.5 1.8 0.6 0.9 ZnO 0.5 0.4 0.6 0.2 1.9 CaO3.4 0.3 0.5 0.4 0.5 BaO 1.5 0.8 1.1 0.7 0.7 TiO₂ 1.9 2.0 1.2 1.7 1.3ZrO₂ 1.9 1.5 1.3 1.3 1.5 AS₂O₃ 0.3 3.2 0.1 3.6 1.8 Sb₂O₃ 0.2 0.4 2.8 0.2V₂O₅ CoO Nucleation temperature(° C.) 680 750 650 650 680Crystallization temperature(° C.) 800 900 750 920 780 Predominantcrystal phase β -Q- β -Q- β -Q- β -Q-SS β -Q-SS SS SS SS Average graindiameter(μm) 0.010 0.010 0.010 0.010 0.010 Predominant crystal phase β -β - β -Eu- Eu- Sp- SS SS SS Average grain diameter(μm) 0.010 0.050 0.010Coefficient of thermal expansion (× 10⁻⁷/° C.) 1 1 10 2 −8 (−60°C.-+600° C.)

[0316] TABLE 43 Examples 8-16 8-17 8-18 8-19 8-20 SiO₂ 55.2 56.8 55.954.1 55.6 P₂O₅ 7.7 6.8 8.1 8.0 8.2 Al₂O₃ 22,7 22.7 23.2 24.0 22.8 Li₂O4.2 4.4 3.4 3.5 3.7 MgO 1.0 1.3 0.8 0.7 0.7 ZnO 1.0 0.5 1.5 0.7 0.4 CaO0.3 3.9 0.7 0.7 3.2 BaO 3.3 0.6 0.5 4.0 1.1 TiO₂ 1.8 1.3 1.3 1.5 1.1ZrO₂ 1.8 1.2 1.5 1.5 1.2 As₂O₃ 1.7 1.3 2.0 Sb₂O₃ 1.0 0.5 1.4 V₂O₅ CoONucleation temperature(° C.) 720 700 750 750 660 Crystallizationtemperature(° C.) 850 760 860 770 760 Predominant crystal phase β -Q β-Q- β -Q- β -Q β -Q-SS SS SS Average grain diameter(μm) 0.007 0.0100.001 0.007 0.001 Predominant crystal phase β - β -Eu- Sp- SS SS Averagegrain diameter(μm) 0.010 0.010 Coefficient of thermal expansion (×10⁻⁷/° C.) 7 7 2 6 7 (−60° C.-+600° C.)

[0317] TABLE 44 Examples 8-21 8-22 8-23 SiO₂ 57.8 59.9 55.1 P₂O₅ 7.6 8.78.2 Al₂O₃ 22.7 22.7 23.0 Li₂O 3.1 3.1 3.4 MgO 0.6 0.6 0.8 ZnO 0.3 0.30.7 CaO 0.4 0.4 2.0 BaO 0.6 0.6 0.7 TiO₂ 3.8 1.3 1.3 ZrO₂ 1.1 1.0 4.0As₂O₃ Sb₂O₃ 2.0 1.4 0.8 V₂O₅ CoO Nucleation temperature (° C.) 650 680740 Crystallization temperature 750 800 940 (° C.) Predominant crystalphase β-Q-SS β-Q-SS β-Q-SS Average grain diameter (μm) 0.001 0.001 0.001Predominant crystal phase β-Sp-SS Average grain diameter (μm) 0.010Coefficient of thermal expansion 5 4 0 (× 10⁻⁷/° C.) (−60° C. − +600°C.)

[0318] TABLE 45 Comparative Examples 9 10 SiO₂ 68.0 76.5 P₂O₅ 2.0 Al₂O₃13.0 3.8 Li₂O 8.0 10.5 MgO 2.5 ZnO 0.5 CaO BaO TiO₂ ZrO₂ 6.0 As₂O₃ 0.5Sb₂O₃ 0.2 V₂O₅ CoO Na₂O 5.0 K₂O 4.0 Nucleation temperature (° C.) 540Crystallization temperature (° C.) 740 Predominant crystal phasechemically lithium disilicate tempered glass α-quartz Coefficient ofthermal expansion 86 80 (× 10⁻⁷/° C.) (−60° C. − +600° C.)

[0319] In the examples shown in Tables 34-44, raw materials such asoxides, carbonates and nitrates were mixed and were melted at atemperature within a range from about 1400° C. to 1500° C. by using aconventional melting device and homogenized by stirring. The melt wasformed and cooled to provide a glass form. It was heat treated under atemperature of 650-750° C. for about 1 to 12 hours for nucleation andfurther heat treated under a temperature of 750-950° C. for about 1 to12 hours for crystallization to provide desired glass-ceramics.

[0320] As shown in Tables 34 to 44, the glass-ceramics of the seventhand eighth aspects of the invention have a grain diameter of theprecipitated crystals within a range of 0.001-0.10 μm.

[0321] As to the crystal phase, the glass-ceramics comprise, as apredominant crystal phase or phases, at least one crystal phase selectedfrom the group consisting of β-quartz (β-SiO₂), β-quartz solid solution(β-SiO₂ solid solution), β-spodumene (β-Li₂O.Al₂O₃.SiO₂), β-spodumenesolid solution (β-Li₂O.Al₂O₃.SiO₂ solid solution), β-eucryptite(β-Li₂O.Al₂O₃.2SiO₂ where a part of Li₂O is replaceable by MgO and/orZnO) and β-eucryptite solid solution (β-Li₂O.Al₂O₃.2SiO₂ solid solutionwhere a part of Li ₂O is replaceable by MgO and/or ZnO), or gahnite(ZnAl₂O₃) and/or gahnite solid solution (ZnAl₂O₃ solid solution).

[0322] As shown in Tables 46-49, the glass-ceramics of the presentinvention differ from the glass-ceramics of the comparative examples inthe crystal phase, that is, the glass-ceramics of the present inventiondo not contain lithium disilicate (Li₂Si₂O₅) but contain at least oneselected from the group consisting of α-cristobalite, α-cristobalitesolid solution, α-quartz and α-quartz solid solution. Further, in theglass-ceramics of Comparative Example No. 11, the average grain diameterof lithium disilicate was 1.5 μm and, in the glass-ceramics ofComparative Example No. 12, the average grain diameter of β-spodumenewas 0.2 μm. These glass-ceramics had relatively large crystal grains ofacicular shape or rice grain shape. Such crystal grains adversely affectsurface roughness after polishing and other factors in the situation inwhich improvement in smoothness is required. The glass-ceramics ofComparative Example Nos. 11 and 12 have surface roughness Ra (arithmeticmean roughness) of 11 or over, showing that it is difficult to obtainexcellent smoothness with surface roughness Ra of 5 Å or below.

[0323] Preferred examples of the ninth aspect of the invention will nowbe described. Tables 46 to 49 show compositions of examples (9-1 to 9-7)and two Li₂O—SiO₂ type glass-ceramics as comparative examples withnucleation temperature, crystallization temperature, crystal phase,average crystal grain diameter, coefficient of thermal expansion withina range of −50-+70° C., specific gravity and surface roughness(arithmetic mean roughness) after polishing. As to the crystal phases inthe tables, α-cristobalite solid solution is expressed as “α-C-SS” andα-quartz solid solution as “α-quartz SS”. TABLE 46 Examples 9-1 9-2 9-3SiO₂ 73.3 75.0 69.2 Li₂O 5.0 5.5 5.0 P₂O₅ 2.0 2.1 2.0 ZrO₂ 2.4 4.0 2.4Al₂O₃ 7.5 7.5 7.5 MgO 0.8 1.8 1.4 ZnO 4.0 0.5 6.0 SrO 1.0 0.6 2.0 BaO1.0 0.5 2.0 Y₂O₃ WO₃ La₂O₃ Bi₂O₃ K₂O 2.0 2.0 2.0 Na₂O Sb₂O₃ 1.0 0.5Nucleation temperature 550 560 540 (° C.) Crystallization 710 750 720temperature (° C.) Predominant crystal phase α-C-SS α-C-SS α-C-SSAverage grain diameter <0.01 μm <0.01 μm <0.01 μm Predominant crystalphase α-quartz SS Average grain diameter  0.01 μm Coefficient of thermal72 110 100 expansion (× 10⁻⁷/° C.) light transmittance (%) 99.0 91.099.0 Young's modulus (GPa) 82 89 81 Bending strength (MPa) 290 400 350Vickers' hardness 760 740 740 Surface roughness (Å) 1.0 2.2 2.0 Specificgravity 2.43 2.48 2.44 Li ion dissolving amount 0.31 0.38 0.28 (μg/disk)(μg/cm²) 0.0046 0.056 0.0041

[0324] TABLE 47 Examples 9-4 9-5 9-6 SiO₂ 63.9 63.9 66.9 Li₂O 6.0 6.06.0 P₂O₅ 2.5 2.5 2.5 ZrO₂ 2.4 2.4 2.4 Al₂O₃ 7.5 7.5 5.5 MgO 2.0 2.0 2.0ZnO 6.0 6.0 6.0 SrO 1.7 1.7 1.7 BaO 2.6 2.6 2.6 Y₂O₃ GeO₂ = 3.0 Gd₂O₃ =3.0 Ga₂O₃ = 2.0 WO₃ La₂O₃ Bi₂O₃ K₂O 2.0 2.0 2.0 Na₂O Sb₂O₃ 0.4 0.4 0.4Nucleation temperature 550 560 540 (° C.) Crystallization 710 750 720temperature (° C.) Predominant crystal phase α-C-SS α-C-SS α-C-SSAverage grain diameter <0.01 μm <0.01 μm <0.01 μm Coefficient of thermal74 100 93 expansion (× 10⁻⁷/° C.) (−50° C. − +70° C.) lighttransmittance (%) 99.0 99.0 99.0 Young's modulus (GPa) 82 89 81 Bendingstrength (MPa) 400 500 450 Vickers' hardness 740 740 740 Surfaceroughness (Å) 1.0 2.2 2.0 Specific gravity 2.45 2.48 2.44 Li iondissolving amount 0.22 0.23 0.19 (μg/disk) (μg/cm²) 0.0033 0.0034 0.0028

[0325] TABLE 48 Examples 9-7 9-8 9-9 SiO₂ 68.2 69.1 69.0 Li₂O 5.0 5.05.0 P₂O₅ 2.0 2.0 2.0 ZrO₂ 2.4 2.4 2.0 Al₂O₃ 7.0 7.0 7.1 MgO 1.4 1.0 1.4ZnO 6.0 7.0 6.0 SrO 2.0 2.0 2.0 BaO 2.0 2.0 2.0 Y₂O₃ 1.0 WO₃ 0.5 Bi₂O₃0.5 K₂O 2.0 2.0 2.0 Na₂O 0.5 As₂O₃ 0.5 0.5 0.5 Nucleation temperature480 470 500 (° C.) Crystallization temperature 715 720 730 (° C.)Predominant crystal phase α-C-SS α-C-SS α-C-SS Average grain diameter<0.01 μm <0.01 μm <0.01 μm Predominant crystal phase α-quartz SS Averagegrain diameter  0.01 μm Coefficient of thermal 85 110 104 expansion (×10−7/° C.) (−50° C. − +70° C.) light transmittance (%) 99.5 92.0 99.5Young's modulus (GPa) 85 98 90 Bending strength (MPa) 300 550 360Vickers' hardness 740 730 760 Surface roughness (Å) 1.0 2.2 2.0 Specificgravity 2.45 2.43 2.46 Li ion dissolving amount 0.32 0.27 0.25 (μg/disk)(μg/cm²) 0.0047 0.0040 0.0037

[0326] TABLE 49 Examples Comparative Examples 9-10 11 12 SiO₂ 69.1 74.276.1 Li₂O 5.0 9.6 11.8 P₂O₅ 2.0 1.5 2.0 ZrO₂ 2.4 0.4 Al₂O₃ 7.0 9.6 7.1MgO 1.0 PbO = 2.3 ZnO 7.0 SrO 1.5 BaO 1.5 WO₃ 0.5 La₂O₃ 0.5 K₂O 2.0 2.42.8 Sb₂O₃ 0.2 As₂O₃ 0.5 Nucleation temperature (° C.) 470 540 500Crystallization 720 800 850 temperature (° C.) Predominant crystal phaseα-C-SS lithium lithium disilicate disilicate Average grain diameter<0.01 μm 1.5 μm 0.1 μm Predominant crystal phase α- α- β-spodumenequartz SS cristobalite Average grain diameter  0.01 μm 0.3 μm 0.2 μmCoefficient of thermal 94 48 49 expansion (× 10⁻⁷/° C.) (−50° C. − +70°C.) light transmittance (%) 97.0 74 60 Young's modulus (GPa) 97 80 86Bending strength (MPa) 600 180 200 Vickers' hardness 750 800 850 Surfaceroughness (Å) 2.0 12 11 Specific gravity 2.50 2.46 2.55 Li iondissolving amount 0.32 3.00 3.80 (μg/disk) (μg/cm²) 0.0047 0.0443 0.0562

[0327] The information storage disk holding members of the inventionhave achieved flatness of 0.1 μm or below after the heating test whichsatisfies the desired flatness (i.e., not greater than 5 μm, preferablynot greater than 3 μm and more preferably not greater than 1 μm). Evenunder a heating temperature of 500° C. or above, they have achieved thedesired flatness (i.e., not greater than 5 νm, preferably not greaterthan 3 μm and more preferably not greater than 1 μm). Depending uponexamples, they have achieved the flatness of the above described desiredrange even under 600° C. for ten minutes, 700° C. for ten minutes and800° C. for ten minutes.

[0328] As described above, the information storage disk holding membersof the invention have excellent heat resisting property. The coefficientof thermal expansion of the glass-ceramics obtained is 2×10⁻⁷−65×10⁻⁷/°C. which is within a suitable range for forming a film of aperpendicular magnetic recording medium.

Industrial Utility

[0329] As described in the foregoing, according to the invention, thereare provided information storage disk holding members, particularlyglass-ceramics spacer rings, which have eliminated the above describeddefects of the prior art and are capable of coping with a high speedrotation of the substrate corresponding to high speed transmission ofinformation, increasing mechanical strength for adaptation to mobileuses, and having a thermal expansion property matching that of otherdrive component parts. There are also provided an information storagedisk drive device on which an information storage disk is mountedthrough these holding members. The information storage disk drive deviceof the present invention can be used for notebook-sized and desktoppersonal computers, servers, mobiles including APS cameras, cellulartelephones, digital cameras, digital video cameras and card drives,storage media of a top box for a network television set and a novel highrecording density media (perpendicular magnetic storage media and islandmagnetic storage media).

1. An information storage disk holding member for holding an informationstorage disk in position, said holding member being made ofglass-ceramics in which a crystal phase is dispersed in a glass matrix.2. An information storage disk holding member as defined in claim 1wherein specific rigidity (Young's modulus/specific gravity) is notsmaller than 37 GPa and specific gravity is not greater than 3.0.
 3. Aninformation storage disk holding member as defined in claim 1 whereinYoung's modulus is within a range from 95 GPa to 130 GPa and specificgravity is within a range from 2.40 to 2.60.
 4. An information storagedisk holding member as defined in claim 1 wherein bending strength iswithin a range from 400 MPa to 800 MPa.
 5. An information storage diskholding member as defined in claim 1 wherein the glass-ceramicscomprise, as a predominant crystal phase or phases, at least one crystalphase selected from the group consisting of lithium disilicate(Li₂O.2SiO₂), α-quartz (α-SiO₂), α-quartz solid solution (α-SiO₂ solidsolution), α-cristobalite (α-SiO₂) and α-cristobalite solid solution(α-SiO₂ solid solution).
 6. An information storage disk holding memberas defined in claim 5 wherein the glass-ceramics comprise, as apredominant crystal phase, lithium disilicate.
 7. An information storagedisk holding member as defined in claim 1 wherein an amount of crystalof lithium disilicate in the glass-ceramics is 3-10 mass % and anaverage crystal grain diameter of the crystal phase is within a rangefrom 0.01 μmm-0.05 μm.
 8. An information storage disk holding member asdefined in claim 1 wherein the glass-ceramics comprise, in mass % onoxide basis, SiO₂  70-79% Li₂O   8-12% K₂O   0-4% MgO   0-less than 2%ZnO   0-less than 2% P₂O₅ 1.5-3% ZrO₂ 1.5-7% Al₂O₃   3-9% Sb₂O₃ + As₃O₃  0-2%.


9. An information storage disk holding member as defined in any ofclaims 1-8 wherein coefficient of thermal expansion within a range from−50° C. to +70° C. is within a range from +65×10⁻⁷/° C. to +130×10⁻⁷/°C.
 10. An information storage disk holding member as defined in claim 1wherein the glass-ceramics comprises, as a predominant crystal phase,α-quartz (α-SiO₂) or α-quartz solid solution (α-SiO₂ solid solution), anamount of the crystal phase is 3-35 mass %, and an average crystal graindiameter of the crystal phase is not greater than 0.10 μm.
 11. Aninformation storage disk holding member as defined in claim 10 whereinan average crystal grain diameter of the entire predominant crystalphase of the glass-ceramics is not greater than 0.05 μm.
 12. Aninformation storage disk holding member as defined in claim 10 whereinthe glass-ceramics are substantially free of PbO.
 13. An informationstorage disk holding member as defined in claim 10 wherein coefficientof thermal expansion within a range from −50° C. to +70° C. is within arange from +95×10⁻⁷/° C. to +110×10⁻⁷/° C.
 14. An information storagedisk holding member as defined in claim 10 wherein the glass-ceramicscomprise, in mass % on oxide basis, SiO₂  70-77% Li₂O   5-less than 9%K₂O   2-5% MgO + ZnO + SrO + BaO   1-2% Y₂O₃+ WO₃ + La₂O₃ + Bi₂O₃   1-3%P₂O₅ 1.0-2.5% ZrO₂ 2.0-7% Al₂O₃   5-10% Na₂O   0-1% Sb₂O₃ + As₃O₃  0-2%.


15. An information storage disk holding member as defined in any ofclaims 10-14 wherein an amount of crystal of lithium disilicate in theglass-ceramics is within a range from 15 mass % to 40 mass %.
 16. Aninformation storage disk holding member as defined in claim 1 whereinthe glass-ceramics comprise, as a predominant crystal phase or phases,at least one crystal phase selected from the group consisting ofcordierite (Mg₂Al₄Si₅O₁₈), cordierite solid solution (Mg₂Al₄Si₅O₁₈ solidsolution), spinel, spinel solid solution, enstatite (MgSiO₃), enstatitesolid solution (MgSiO₃ solid solution), β-quartz (β-SiO₂), β-quartzsolid solution (β-SiO₂ solid solution), magnesium titanate ((MgTi₂O₅)and magnesium titanate solid solution (MgTi₂O₅ solid solution).
 17. Aninformation storage disk holding member as defined in claim 16 whereinthe glass-ceramics comprise, in mass % on oxide basis, SiO₂  40-60% MgO 10-18% Al₂O₃  10-less than 20% P₂O₅   0-4% B₂O₃   0-4% CaO 0.5-4% SrO  0-2% BaO   0-5% ZrO₂   0-5% TiO₂ 2.5-12% Bi₂O₃   0-6% Sb₂O₃   0-1%As₂O₃   0-1% Fe₂O₃   0-2%.


18. An information storage disk holding member as defined in claim 16 or17 wherein coefficient of thermal expansion within a range from −50° C.to +70° C. is within a range from +30×10⁻⁷/° C. to +65×10⁻⁷/° C.
 19. Aninformation storage disk holding member as defined in claim 1 whereinthe glass-ceramics comprise, as a predominant crystal phase or phases,at least one crysal phase selected from the group consisting ofenstatite (MgSiO₃), enstatite solid solution (MgSiO₃ solid solution),magnesium titanate (MgTi₂O₅), magnesium titanate solid solution (MgTi₂O₅solid solution), spinel and spinel solid solution, the glass-ceramicscomprise Al₂O₃ in an amount of less than 20 mass %, and theglass-ceramics have Young's modulus within a range from 115 GPa to 160GPa.
 20. An information storage disk holding member as defined in claim19 wherein the glass-ceramics comprise enstatite (MgSiO₃) or enstatitesolid solution (MgSiO₃ solid solution) as a crystal phase having thelargest precipitation amount First phase).
 21. An information storagedisk holding member as defined in claim 19 wherein the glass ceramicscomprise magnesium titanate (MgTi₂O₅) or magnesium titanate solidsolution ((MgTi₂O₅ solid solution) as a crystal phase having the largestprecipitation amount first phase).
 22. An information storage diskholding member as defined in claim 20 wherein the glass-ceramicscomprise, as a crystal phase having a precipitation amount which issmaller than the precipitation amount of the first phase, at least onecrystal phase selected from the group consisting of magnesium titanate(MgTi2O₅), magnesium titanate solid solution (MgTi₂O₅ solid solution),spinel and spinel solid solution.
 23. An information storage diskholding member as defined in claim 21 wherein the glass-ceramicscomprise, as a crystal phase having a precipitation amount which issmaller than the precipitation amount of the first phase, at least onecrystal phase selected from the group consisting of enstatite (MgSiO₃),enstatite solid solution (MgSiO₃ solid solution), spinet and spinetsolid solution.
 24. An information storage disk holding member asdefined in claim 19 wherein the glass-ceramics are substantially free ofLi₂O, Na₂O and K₂O.
 25. An information storage disk holding member asdefined in claim 19 wherein the glass-ceramics comprise, in mass % onoxide basis, SiO₂  40-60% MgO  10-20% Al₂O₃  10-less than 20% CaO 0.5-4%SrO 0.5-4% BaO   0-5% ZrO₂   0-5% TiO₂ exceeding 8% and up to 12% Bi₂O₃  0-6% Sb₂O₃   0-1% As₂O₃   0-1%.


26. An information storage disk holding member as defined in claim 19wherein the glass-ceramics comprise an element selected from P, W, Nb,La, Y and Pb in an amount of up to 3 mass % on oxide basis and/or anelement selected from Cu, Co, Fe, Mu, Cr, Sn and V in an amount of up to2 mass % on oxide basis.
 27. An information storage disk holding memberas defined in claim 19 wherein coefficient of thermal expansion within arange from −50° C. to +70° C. is within a range from +40×10⁻⁷/° C. to+60×10⁻⁷/° C.
 28. An information storge disk holding member as definedin claim 19 wherein a crystal grain diameter of the respective crystalphases is within a range from 0.05 μm 0.30 μm.
 29. An informationstorage disk holding member as defined in any of claims 19-28 whereinVickers' hardness is within a range from 700 to
 850. 30. An informationstorage disk holding member as defined in claim 1 wherein a predominantcrystal phase or phases of the glass-ceramics are at least one crystalphase selected from the group consisting of β-quartz, β-quartz solidsolution, enstatite, enstatite solid solution, forsterite andforesterite solid solution.
 31. An information storage disk holdingmember as defined in claim 30 wherein the glass-ceramics comprise Al₂O₃in an amount within a range from 10 mass % to less than 20 mass % onoxide basis and have Young's modulus (GPa)/specific gravity within arange from 37 to
 63. 32. An information storage disk holding member asdefined in claim 30 wherein the glass-ceramics comprise, in mass % onoxide basis, SiO₂   40-60% MgO   10-20% Al₂O₃   10-less than 20% P₂O₅ 0.5-2.5% B₂O₃    1-4% Li₂O  0.5-4% CaO  0.5-4% ZrO₂  0.5-5% TiO₂ 2.5-8% Sb₂O₃ 0.01-0.5% As₂O₃    0-0.5% SnO₂    0-5% MoO₃    0-3% CeO   0-5% Fe₂O₃    0-5%/


33. An information storage disk holding member as defined in claim 30wherein the glass-ceramics are substantially free of Na₂O, K₂O and PbO.34. An information storage disk holding member as defined in any ofclaims 30-33 wherein a crystal grain diameter of the respective crystalphases is within a range from 0.05 μm to 0.30 μm.
 35. An informationstorage disk holding member as defined in claim 1 wherein theglass-ceramics comprise, as a predominant crystal phase or phases, atleast one crystal phase selected from the group consisting ofcordierite, cordierite solid solution, spinel, spinel solid solution,enstatite, enstatite solid solution, β-quartz and β-quartz solidsolution.
 36. An information storage disk holding member as defined inclaim 1 wherein the glass-ceramics comprise, as a predominant crystalphase or phases, at least one crystal phase selected from the groupconsisting of β-quartz, β-quartz solid solution, enstatite, enstatitesolid solution, forsterite and foresterite solid solution.
 37. Aninformation storage disk holding member as defined in claim 35 wherein acrystal grain diameter of the respective crystal phases is within arange from 0.05 μm to 0.30 μm.
 38. An information storage disk holdingmember as defined in claim 35 wherein the glass-ceramics comprise, inmass % on oxide basis, SiO₂  40-60% MgO  10-20% Al₂O₃  10-less than 20%P₂O₅   0-4% B₂O₃   0-4% CaO 0.5-4% BaO   0-5% ZrO₂   0-5% TiO₂ 2.5-8%Sb₂O₃   0-1% As₂O₃   0-1% F   0-3% Fe₂O₃   0-5%.


39. An information storage disk holding member as defined in claim 35wherein the glass-ceramics have Young's modulus (GPa)/ specific gravitywithin a range from 37 to 63 and comprise Al₂O₃ within a range from 10%to less than 20%.
 40. An information storage disk holding member asdefined in claim 35 wherein the glass-ceramics are substantially free ofNa₂O, K₂O and PbO.
 41. An information storage disk holding member asdefined in any of claims 35-40 wherein coefficient of thermal expansionwithin a range from −50° C. to +70° C. is within a range from +30×10⁻⁷/°C. to +50×10⁻⁷/° C.
 42. An information storage disk holding member asdefined in claim 1 wherein the glass-ceramics comprise, as a predominantcrystal phase or phases, at least one crystal phase selected from thegroup consisting of β-quartz (β-SiO₂), β-quartz solid solution (β-SiO₂solid solution), β-spodumene (β-Li₂O.Al₂O₃.SiO₂), β-spodumene solidsolution (β-Li₂O.Al₂O₃.SiO₂ solid solution), β-eucryptite(β-Li₂O.Al₂O₃.2SiO₂ where a part of Li₂O is replaceable by MgO and/orZnO) and β-eucryptite solid solution (β-Li₂O.Al₂O₃.2SiO₂ solid solutionwhere a part of Li₂O is replaceable by MgO and/or ZnO).
 43. Aninformation storage disk holding member as defined in claim 42 whereinan average crystal grain diameter of the glass-ceramics is within arange from 0.001 μm to 0.10 μm.
 44. An information storage disk holdingmember as defined in claim 42 wherein the glass-ceramics comprise, inmass % on oxide basis, SiO₂  50-62% P₂O₅   5-10% Al₂O₃  22-26% Li₂O +MgO + ZnO   4-6.5% in which Li₂O   3-5% MgO 0.5-2% ZnO 0.2-2% CaO + BaO0.8-5% in which CaO 0.3-4% BaO 0.5-4% TiO₂   1-4% ZrO₂   1-4% As₂O₃ +Sb₂O₃   0-4% and are substantially free of PbO, Na₂O and K₂O.


45. An information storage disk holding member as defined in any ofclaims 42-44 wherein coefficient of thermal expansion within a rangefrom −50° C. to +600° C. is within a range from −10×10⁻⁷/° C. to+20×10⁻⁷/° C.
 46. An information storage disk holding member as definedin claim 1 wherein a predominant crystal phase of the glass-ceramics isgahnite (ZnAl₂O₃) and/or gahnite solid solution (ZnAl₂O₃ solidsolution).
 47. An information storage disk holding member as defined inclaim 46 wherein the glass-ceramics are substantially free of PbO, Na₂Oand K₂O.
 48. An information storage disk holding member as defined inclaim 46 wherein the glass-ceramics comprise, in mass % on oxide basis,SiO₂  30-65% Al₂O₃   5-35% ZnO   5-35% MgO   1-20% TiO₂   1-15% CaO +SrO + BaO + B₂O₃+ La₂O₃ + Y₂O₃ + 0.5-20% Gd₂O₃ + Ta₂O₅ + Nb₂O₅ + WO₃ +Bi₂O₃ in which B₂O₃   0-10% and Ta₂O₅ + Nb₂O₅ + WO₃ + Bi₂O₃   0-10%ZrO₂ + P₂O₅ + SnO₂   0-7% in which ZrO₂   0-less than 2% P₂O₅   0-5%SnO₂   0-2% As₂O₃ + Sb₂O₃   0-4%


49. An information storage disk holding member as defined in any ofclaims 46-48 wherein coefficient of thermal expansion within a rangefrom −50° C. to +600° C. is within a range from +35×10⁻⁷/° C. to+65×10⁻⁷/° C.
 50. An information storage disk holding member as definedin claim 1 wherein the glass-ceramics comprise, as a predominant crystalphase or phases, at least one crystal phase selected from the groupconsisting of α-cristobalite, α-cristobalite solid solution, α-quartzand α-quartz solid solution but are substantially free of lithiumdisilicate (Li₂O.2SiO₂), lithium silicate (Li₂O.SiO₂), β-spodumene,β-eucryptite, β-quartz, mica and fluorrichterite and also are free of Crand Mn, have a coefficient of thermal expansion within a range from −50°C. to +70° C. which is within a range from +65×10⁻⁷/° C. to +140×10⁻⁷/°C. and have an average crystal grain diameter of the predominant crystalphase of less than 0.10 μm.
 51. An information storage disk holdingmember as defined in claim 50 wherein the glass-ceramics have Young'smodulus which is not smaller than 80 GPa.
 52. An information storagedisk holding member as defined in claim 50 wherein the glass-ceramicshave specific gravity within a range from 2.3 to 2.7.
 53. An informationstorage disk holding member as defined in claim 50 wherein theglass-ceramics have light transmittance for a plate thickness of 10 mmwhich is 90% or over within a wavelength range from 950 nm to 1600 nm.54. An information storage disk holding member as defined in claim 50wherein the glass-ceramics have bending strength of 250 MPa or over. 55.An information storage disk holding member as defined in claim 50wherein the glass-ceramics have Vickers' hardness within a range from600 to
 800. 56. An information storage disk holding member as defined inany of claims 50-55 wherein the glass-ceramics comprise, in mass % onoxide basis, SiO₂  65-75% Li₂O   4-less than 7% K₂O   0-3% Na₂O   0-3%MgO + ZnO + SrO + BaO + CaO   2-15% Y₂O₃ + WO₃ + La₂O₃ + Bi₂O₃   0-3%SnO₂   0-3% P₂O₅ 1.0-2.5% ZrO₂ 2.0-7% Al₂O₃   5-9% Sb₂O₃ + As₂O₃   0-1%.


57. An information storage disk holding member as defined in any ofclaims 1, 10 and 16 wherein the glass-ceramics are obtained bysubjecting base glass obtained by melting and forming glass rawmaterials to heat treatment for nucleation under a temperature within arange from 400° C. to 600° C. for one to seven hours and furthersubjecting the base glass to heat treatment for crystallization under atemperature within a range from 700° C. to 780° C. for one to sevenhours.
 58. An information storage disk holding member as defined in anyof claims 19, 30, 35, 36, 42 and 46 wherein the glass-ceramics areobtained by subjecting base glass obtained by melting and forming glassraw materials to heat treatment for nucleation under a temperaturewithin a range from 650° C. to 750° C. for one to seven hours andfurther subjecting the base glass to heat treatment for crystallizationunder a temperature within a range from 750° C. to 950° C. for one toseven hours.
 59. An information storage disk holding member as definedin claim 50 wherein the glass-ceramics are obtained by subjecting baseglass obtained by melting and forming glass raw materials to heattreatment for nucleation under a temperature within a range from 400° C.to 600° C. for one to seven hours and further subjecting the base glassto heat treatment for crystallization under a temperature within a rangefrom 650° C. to 750° C. for one to seven hours.
 60. An informationstorage disk holding member made by forming a conductive film on thesurface of the holding member as defined in any of claims 1, 10, 16, 19,30, 35, 36, 42, 46 and
 50. 61. A spacer ring for an information storagedisk made of the holding member as defined in any of claims 1, 10, 16,19, 30, 35, 36, 42, 46 and 50, said holding member having a ring shape.62. An information storage disk drive device capable of holding one ormore information storage disks on a rotor hub by means of the spacerring as defined in claim
 61. 63. An information storage disk drivedevice as defined in claim 62 wherein the rotor hub and the spacer ringhave a coefficient of thermal expansion which is substantially equal tothat of the information storage disk.